Evaluation of the corrosion and mechanical properties of a range of experimental CrMn stainless steels

Evaluation of the corrosion and mechanical properties of a range of experimental CrMn stainless steels

MATERIALS SCIENCE & ENGINEERING ELSEVIER Materials Science and Engineering A199 (1995) 183 194 A Evaluation of the corrosion and mechanical propert...

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MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A199 (1995) 183 194

A

Evaluation of the corrosion and mechanical properties of a range of experimental C r - M n stainless steels M. Kemp, A. van Bennekom, F.P.A. Robinson Department of Metallurgy, University of the Witwatersrand, Private Bag 3, Johannesburg, Wits 2050, South A/?ica Received 15 July 1994; in revised form 16 September 1994

Abstract

In this resumed investigation, the effectiveness of manganese as an austenite-forming element in 17% and 20% chromium stainless steels has been investigated. The effect of manganese on the corrosion and mechanical properties of a range of experimental, low-cost, manganese-containing, dual-phase stainless steels is evaluated and reported. Metallographic examination and image analysis of the alloys revealed that manganese contents of up to 9%, in both 17% and 20% chromium alloys, are effective in producing a dual-phase structure comprising ferrite and austenite/martensite. It was found that manganese acts as an austenite-forming element under the conditions of the investigation. Mechanical testing of the alloys included hardness, tensile and Charpy impact testing. The results of the hardness testing showed that a 1050 °C normalizing treatment, followed by a 600 °C heat treatment and water quenching, resulted in alloys in their softest condition. It was found that manganese additions increased the toughness of the stainless steels, while tensile testing revealed that manganese additions increased the ultimate tensile strength of the alloys, but had little effect on the yield strength. The addition of manganese was found to reduce both the general and pitting corrosion resistances. Under reducing conditions, manganese additions resulted in a dramatic deterioration of the corrosion properties of the alloys.

Keywords: Corrosion; Mechanical properties; C r - M n stainless steels

1. Introduction

Stainless steels are extensively used in industry mainly because of their high resistance to corrosion. However, due to their high cost, stainless steels cannot be used in m a n y applications where they would otherwise be an ideal choice, F o r this reason, attempts are being made all over the world to produce cheaper stainless steels, whilst still maintaining their high corrosion resistance. Rather costly austenitic stainless steels are the most extensively used stainless steels because of their excellent corrosion resistance coupled with ease of weldability and formability. The high cost of austenitic stainless steels stems from their relatively high nickel content (8% or more). Ferritic steels, on the other hand, do not contain Ni and are thus much cheaper than the austenitic grades. The ferritic stainless steels, however, suffer from poor weldability and as such can only be welded in thin sections. 0921-5093/95/$09.50 © 1995 - - Elsevier Science S.A. All rights reserved SSDI 0921-5093(94)09694-5

The combination of these two grades of stainless steel results in duplex stainless steels which combine the corrosion resistance and weldability of austenitic stainless steels with the lower cost of ferritic stainless steels. However, duplex stainless steels do contain some nickel (4%-6%) and are thus still relatively expensive. Thus if a nickel-free duplex stainless steel could be produced, the cost would be further reduced. The possibility of replacing nickel with manganese to produce a cheap duplex stainless steel is reported in this paper. Stainless steels generally do not contain large amounts of manganese, with most alloys containing below 1% [1]. Manganese is generally added to steels to combine with sulphur to form manganese sulphide inclusions, thereby preventing problems during hot working and also preventing the formation of deleterious chromium sulphides. Sulphur is usually present in stainless steels at levels of about 0.03%. Since the solubility of sulphur in stainless steel is about 0.01% at

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M. Kemp et al. / Materials Science and Engineering A199 (1995) 183-194

Table 1 Chemical compositions of the alloys investigated Alloy 5581 5591 5601 5611 5621 5631 5641 5651

Mn (%)

Si (%)

Ni (%)

C

N

Mo (%)

S

(%)

Cu (%)

P

(%)

(%)

(%,)

Ti (%)

Mb (%)

3.14 5.19 7.43 9.45 3.17 5.32 7.55 9.41

0.53 0.44 0.40 0.32 0.54 0.53 0.52 0.47

0.40 0.40 0.38 0.38 0.35 0.34 0.34 0.33

0.022 0.023 0.021 0.021 0.031 0.031 0.030 0.027

0.028 0.031 0.029 0.026 0.031 0.031 0.032 0.032

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

0.023 0.022 0.022 0.021 0.022 0.021 0.021 0.020

0.004 0.005 0.004 0.004 0.006 0.003 0.004 0.004 0.004 0.003 0.004 0.004 0.005 0.006 0.003 0.005 0.005 0.003 0.006 0.005 0.003 0.004 0.004 0.003

V (%)

Cr (%)

0.09 0.09 0.10 0.11 0.11 0.11 0.12 0.12

16.54 17.09 16.38 16.98 16.04 16.60 16.00 16.52 19.61 20.20 19.16 19.76 18.64 19.25 18.29 18.90

PRE a

CrEQb 17.82 17.52 17.17 17.06 21.00 20.54 20.05 19.63

~PRE, %Cr + 3.3(%Mo) + 16(%N). bCrEQ, %Cr + 1.5(%Si)+ %Mo + 0.5(%Nb) + 3(°A,Al)+ 5(%V). room temperature, sulphur usually exists in the form of the aforementioned inclusions [2]. According to Wilde and Armijo [3], sulphur increases the anodic dissolution rate at potentials more active than the primary passivation potential. Hoar and Havenhand [4] have reported that sulphur in mild steel has a deleterious effect on the corrosion resistance in weak acidic media. Thus it is obvious that sulphur levels should be kept as low as possible to prevent its detrimental effects on the corrosion properties. According to Henthorne [5], sulphide inclusions have a detrimental effect on the pitting resistance in all grades of stainless steel. The extent to which the inclusions act as pit initiation sites depends on the manganese and chromium contents as well as the chemical treatments aimed at removing them from the surface [2]. The reason why high levels of manganese are not generally added to stainless steels is because it increases the rate of wear of the refractories during the steel making process. Low sulphur levels (0.006% 0.01%) can be obtained with argon oxygen decarburization (AOD) practice; however, if the sulphur level is too low, it has an adverse effect on machinability. Sulphur is intentionally added to some austenitic grades to levels of about 0.3% to improve machinability, e.g. Type 303 [2]. Austenitic F e - C r M n - N i - N (AISI 200 Series) stainless steels have corrosion resistances corresponding to those of the AISI 300 Series during atmospheric exposure and under oxidizing conditions [6]. The F e C r - M n and F e - C r - M n - N steels are, however, inferior to the 300 Series with respect to general corrosion resistance especially under reducing conditions [7]. The reason for this is that chromium and nickel have beneficial effects on the corrosion resistance of steels, while manganese is generally seen to have a detrimental effect on the general corrosion resistance of stainless steels [8]. The effect of manganese on the pitting resistance of F e - C r - M n alloys is rather controversial with various effects being observed. Lunarska et al. [9] believe that this controversy is due mainly to the fact that these steels are extremely sensitive to the presence of minor

constituents such as carbon, nitrogen, sulphur and phosphorus. The effect of manganese on the microstructure of stainless steels is also controversial, with various researchers presenting conflicting viewpoints on this subject. The main issue of controversy is whether manganese acts as an austenite or a ferrite former. Traditionally, manganese is accepted as being an austenite former, albeit a weak one, with existing phase diagrams showing that manganese forms austenite. Some researchers, however, have shown that manganese acts as a ferrite-forming element [10,11]. The literature on the mechanical properties indicates that manganese additions alone do not greatly influence the mechanical properties of Fe C r - M n alloys provided that phase changes do not occur. However, nitrogen additions together with manganese additions have a beneficial effect, improving the strength and toughness of stainless steels [12].

2. E x p e r i m e n t a l p r o c e d u r e

Eight experimental alloy stainless steel plates, supplied and produced by Columbus Stainless, containing various amounts of manganese and chromium (presented in Table 1), were first heat treated at 1050 °C for 1 h, followed by water quenching. A subsequent 600 °C heat treatment for 1 h, followed by water quenching was chosen to produce fully soft alloys. All of the treated samples were then cut into small specimens for metallography and corrosion testing purposes. 2. I. Metallographic procedure

Metallographic samples were prepared by mounting and polishing to a 0.5 gm finish using alumina paste. Different etchants were then applied to the samples to enhance different microstructural features depending on the experimental technique being carried out. For metallographic examination, two etchants were used to highlight different features in the specimens.

M. Kemp et al. / Materials" Science and Engineering A199 (1995) 183 194

The general microstructure of the specimen was shown by using a dip etch prepared by mixing dilute HC1 (1 : 5) with 2 g of N H 4 F . H F and 1 g of K2SO 4. The samples were dipped in this solution for 8 s prior to metallographic and scanning electron microscopy (SEM) examinations. The samples were repolished and were then electrolytically etched in 10% oxalic acid for 20 s at 7 V to highlight the precipitates in the samples.

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in both deaerated 0.1 N H2SO 4 and 6.7 N acetic acid solutions held at 25 °C to evaluate the corrosion performances in reducing and oxidizing environments respectively. The samples were first conditioned at a potential of - 7 0 0 mV below the free corrosion potential for 600 s to remove the passive film, after which the scanning was initiated at this potential and terminated at a potential of 1600 mV using an anodic scan rate of 1 mV s

2.2. Image analysis 2.6. Pitting corrosion testing (cyclic polarization) The relative percentages of martensite/austenite and ferrite in the samples were determined by image analysis. The phase percentage of precipitates in some samples was also determined after etching in 10% oxalic acid as previously described. A 10 N K O H electrolytic tint etch was used to produce sufficient colour contrast between ferrite and martensite/austenite for image analysis. This etchant was applied by anodically polarizing the sample for 25 s at 1.8 V. This resulted in the ferrite appearing as the dark phase in the microstructure and the austenite/ martensite appearing as the light phase. Image analysis was performed using a magnification of 100 x with at least 25 fields being measured on each sample.

2.3. Scanning electron microscopy Both tensile and Charpy fracture surfaces, as well as corrosion samples, were examined by SEM after testing. After pitting corrosion testing, the samples were lightly etched so as to indicate whether preferential pitting corrosion occurred in a particular phase. Precipitates were also analysed using the energy dispersive analysis of X-rays (EDAX) facility.

2.4. Transmission electron microscopy (TEM) sample preparation Test specimens were machined to 3 m m rod-shaped specimens from the 17% chromium alloys containing 3% and 9% manganese. The machined rods were then cut into 0.2 m m slices using a Polaron lathe and an SiC cutting wheel. The discs were then progressively wet ground on both sides to a final grit size of 1000. An optically controlled Fishione TM twin jet electropolishing system was then used to polish the samples to perforation in a mixture of acetic acid, 6% (w/w) perchloric acid and 0.1% (w/w) CrO3 at 25 °C. The applied potential was set at 27 or 75 V and the current at 180 mA. A 200 kV Philips 20C transmission electron microscope was then used to view the samples.

2.5. General corrosion testing (potentiodynamic) Potentiodynamic polarization scans were performed

Pitting corrosion samples were progressively ground down to 600 grit sand paper and then placed in 55% nitric acid at 50 °C for 30 min in accordance with the sample preparation technique proposed by Lee et al. [13]. The samples were then well rinsed in water and the edges of the exposed sample were masked with an epoxy resin (Pratley Quickset TM) to reduce the possibility of crevice corrosion between the sample and the fibreglass resin. Alumina paste and a brush were then used to remove the passive film from the surface of the specimen being tested, with the result that the passive film remained intact around the edges of the specimen, thus reducing the chance of crevice corrosion. The samples were then tested in deaerated 0.025 N NaC1 solution held at 25 °C. Before the scan was initiated, the samples were allowed to remain in the pitting solution for 1 h so as to reach their free corrosion potential. A scan rate of 1 mV s 1 was used with the potential reversal occurring at 500 laA cm 2.

2. 7. Hardness testing Vickers hardness testing was performed on all the samples using a test mass of 20 kg and a dwell time of 5 s. Five tests were performed on each of the samples to obtain a satisfactory average.

2. 8. Mechanical properties testing The influence of manganese on the mechanical properties of the stainless steel alloys was investigated using room-temperature Charpy V-notch impact and tensile tests. Three Charpy and three tensile test specimens were machined from the steel plates after heat treatment.

3. Results and discussion

3.1. Hardness testing Before commencing the corrosion and mechanical testing, it was necessary to ensure that all the alloys were in the same condition. For this reason, hardness

M. Kemp et al. / Materials Science and Engineering A199 (1995) 183 194

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tests were conducted on the 17% chromium alloys containing various amounts of manganese and heat treated at different temperatures for 1 h followed by water quenching. F r o m the results shown in the graph in Fig. 1, it can be seen that low hardness values were obtained after the alloys were heat treated at 600 °C for 1 h and water quenched. The hardnesses after the 1050 and 1200 °C heat treatments were also low, since these temperatures are above the Acl temperature (900 °C) and the samples would probably contain some austenite at room temperature. Apart from the retention of the softer austenite to r o o m temperature, the hardnesses will also be reduced by the smaller amount of martensite formed due to the retention of austenite. The 600 °C heat treatment showed the lowest hardness values for all of the manganese contents investigated and it was therefore decided to test the alloys in this, the softest, condition. Following this heat treatment, the microstructures of the 17% chromium alloys were found to comprise a mixed ferrite-tempered martensite-retained austenite structure. A similar set of tests was conducted on the 20% chromium alloys and the results are shown in the graph in Fig. 2. The additional 3% chromium in these alloys results in a predominantly ferritic structure. These hardness curves show very little change with temperature as would be expected for essentially single-phase ferritic alloys. F r o m these results and those obtained from the previous set of tests, it was decided to test the 20% chromium alloys after heat treatment at 600 °C for 1 h followed by water quenching.

3.2. Metallography and phase percentage results Metallographic examinations performed on the specimens after various heat treatments revealed that the general structure of most of the specimens included ferrite, retained austenite and martensite. Fig. 3 shows 350 300! . . . . . . . . . . . . . . . . . . . . . . > 2

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Fig. 2. Effects of heat treatment temperature and manganese content on the Vickers hardness of the 20% chromium alloys after being held at the treatment temperature for l h and water quenched. the microstructures of the 17% chromium alloys containing increasing amounts of manganese. From this set of micrographs, it can be seen that an increase in manganese addition results in an increase in the amount of austenite/martensite phase (light phase). Fig. 3(a) clearly shows that the ferrite forms first on solidification, with the austenite growing into the ferrite from the ferrite grain boundaries by the Widmanstfitten growth mechanism. Elongated manganese sulphide inclusions (black) can also be seen along the grain boundaries of these alloys. T E M was used to determine the crystal structure of the martensite found in the 17% chromium samples. F r o m the diffraction pattern in the inset of Fig. 4, it was calculated that the martensite was c~-martensite having a b.c.c, structure. It can also be seen in Fig. 4 that the grain boundaries between the ferrite (F) and martensite (M) phases are preferentially attacked. This preferential corrosion probably occurs as a result of grain boundary precipitates present between the two phases. The ferrite-ferrite grain boundaries are, by contrast, comparatively free of precipitates, as can be seen in the bottom left-hand corner. The reason for the preferential precipitation along the ferrite-austenite grain boundaries is probably that the austenite grains have a higher solubility for nitrogen and carbon than the ferrite grains. This results in a concentration gradient being set up between the ferrite and austenite grains. At high temperatures, the nitrogen and carbon diffuse into the ferrite and, on cooling, form precipitates along the ferrite-austenite grain boundaries. As a result of this, the precipitates are seen to extend into the ferrite grains. The percentages of each of the phases present in the alloys were determined using a combination of image analysis and magnetic phase measurements. The results of the phase percentage determinations from the image analyser, presented graphically in Fig. 5, show that a peak in the austenite/martensite phase percentage is

M. Kemp et al. / Materials" Science and Engineering AI99 (1995) 183-194

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observed when the alloys are quenched from 900 °C. This peak is a result of the fact that the alloy is in the austenite region, "7 loop", of the phase diagram at this temperature. As the manganese content of the alloys is increased from 3% to 9%, the peak becomes more pronounced, as a result of the manganese additions, which expand the range of the 7 loop resulting in the formation of more austenite at high temperature. From the F e - C r binary phase diagram for 3% manganese steels, shown in Fig. 6, it can be seen that a 17% chromium alloy would not be fully austenitic at 900 °C. From this it is obvious that the 20% chromium alloy would contain even less austenite by virtue of the fact that chromium is a strong ferrite former. Fig. 5 shows that, at a chromium content of 20% and manganese contents of 3% and 5%, the phase percentage of austenite/martensite varies only slightly with heat treatment temperature. This is probably because, at these low manganese contents, there is not enough of this element to extend sufficiently the 7 loop to the 17% chromium level and thus very little austenite or martensite is formed. At manganese contents in excess of 7%,

clearly definable peaks in the austenite/martensite phase percentages are observed when these alloys are held in the temperature region corresponding to the nose of the ?, loop (900 °C). It can be seen from Fig. 7 that less austenite/martensite is present at room temperature as the chromium content of the alloy is increased from 17% to 20%. From this figure, it appears that, at low nitrogen and carbon contents, manganese acts as an austenite former. This contradicts the results of Hull [14] who found that manganese contents above 2.5% may lead to manganese exhibiting ferrite-forming characteristics in 12% Cr steels. Szumachowski and Kotecki [15] and Ritter et al. [16] also found that, under their conditions, manganese acted as a ferrite former. However, in stainless steels, manganese is generally accepted to be an austenite former, which is confirmed by the results in this report. According to Hochmann [17], certain ranges of manganese addition enhance the formation of austenite at high temperatures, while certain ranges of nitrogen addition enable the austenitic structure to be retained

(a}

(b)

Fig. 3. (a), (b).

188

M. Kemp et al. / Materials Sc&nce and Engineering A199 (1995) 183-194

(c)

(d)

Fig. 3. The effects of increasing manganese content on the microstructure of the 17% chromium alloys after heat treatment at 600 °C and water quenching (magnification 200 × ): (a) 3% Mn; (b) 5% Mn; (c) 7% Mn; (d) 9% Mn.

to room temperature. Thus it would be expected that manganese in combination with nitrogen in the steel would have similar effects to those of nickel in the stabilization of austenite. The volume fractions of precipitates in some samples were analysed using image analysis after electrolytic etching in 10% oxalic acid. From the graph in Fig. 8, it is obvious that an increase in the manganese content results in a decrease in the amount of precipitates. This is due to the fact that an increase in manganese content stabilizes austenite which has a much higher solubility for carbon and nitrogen. As a result, the solubility of carbon and nitrogen in the steel increases and reduces the amount of precipitates. As expected, the 20% chromium samples which contain more ferrite (which has a much lower solubility for carbon and nitrogen) have a higher percentage of precipitates than the 17% chromium alloys which contain martensite and austenire. This effect can clearly be seen in Fig. 8 which shows that, even at 9% manganese, the 20% chromium alloys contain more precipitates than the 17% chromium alloys with only 3% manganese.

3.3. Mechanical properties results The effect of the manganese and chromium contents on the mechanical properties of the stainless steels was investigated after heat treatment. As mentioned previously, heat treatment at 600 °C followed by water quenching results in a soft ferrite-tempered martensite-retained austenite structure, making this heat treatment the most promising for mechanical properties testing. The results of the impact toughness testing, performed using a Charpy "V" notch test, are given in Table 2 and Fig. 9. From the graph in Fig. 9 it can be seen that, as the manganese content increases, the toughness of the alloys increases. This is because the increasing manganese content causes an increase in the phase percentage of the more ductile austenite phase, as reported previously. The presence of a second phase, such as austenite or martensite, results in a refinement of the grain size, referred to as transformation grain refinement, and leads to an increase in strength and ductility as described by the H a l l - P e t c h relationship. In

Materials Science and Engineering A199 (1995) 183 194

M. Kemp et al.

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Fig. 4. Transmission electron micrograph of the 9%Mn-17%Cr alloy after water quenching from 600 °C, showing the presence of ~martensite (magnification 6610 x ). Inset shows diffraction pattern.

the 20% chromium alloys, increasing the manganese content from 3% to 5% has little effect on the toughness; however, a large increase in the impact toughness occurs when the manganese content is increased from 5% to 7%. This can be explained by the fact that, when more chromium is present, a larger amount of austenite formers will have to be added to expand the 7 loop to higher chromium levels before a second phase starts to form. This result indicates that the microstructure is virtually fully ferritic up to a manganese content of 5%.

This result was confirmed by the metallographic examination. The 17% chromium alloys do not show this effect to the same extent because much less manganese is required to expand the 7 loop to this chromium level. It must be noted that the results of these impact tests are very low and the resulting fracture surfaces are predominantly brittle. SEM was used to examine the fracture surfaces of the impact test specimens; the extent of brittle fracture observed in all of the low manganese specimens is shown in the electron micrograph in Fig. 10. The scale marker should be noted for full appreciation o f the extent of the brittle nature of the fracture surface. 50

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Fig. 7. Effects of chromium and manganese contents on the austenite and martensite phase percentages of alloys held at 600 °C for 1 h and water quenched.

M. Kemp et al. / Materials Science and Engineering A199 (1995) 183-194

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[ -- 17%0r -'- 2o%Cr ] Fig. 8. Effects of chromium and manganese contents on the percentage of precipitates in the alloys held at 600 °C for 1 h and water quenched.

Fig. 9. The effects of manganese and chromium on the toughness of the experimental alloys after being held at 600 °C for 1 h and then water quenched.

As the manganese content of the alloys is increased, small ductile regions, in the form of microvoids, are observed on the fracture surfaces, as shown in Fig. 11. Precipitates found in these voids were examined using energy dispersive spectroscopy (EDS) and were found to be predominantly manganese sulphide inclusions. These inclusions were easily recognized by their elongated shape and were often fragmented as a result of impact testing. The T E M investigation showed these inclusions to be present in all of the samples in both the ferrite and austenite/martensite phases (see Fig. 12). As the manganese content of the alloys is increased, the extent of the ductile regions on the fracture surfaces increases resulting in improved impact toughness values shown in the graph in Fig. 9. Decohesion between the grains, as shown in Fig. 13, also contributes towards the improved toughness of the high manganese alloys. The results of impact testing are in agreement with those of Kato et al. [18], who studied the energies absorbed in Charpy 2 mm "V" notch impact tests in F e - C r - M n alloys with 5% Cr, and found that increasing manganese contents led to increased impact energy. This can be related to the austenite-forming effect of

manganese. However, the results of these tests contradict those obtained by Neef [19], who studied the effect of the manganese content on the impact toughness of an alloy of the following composition: 15% Ni, 15% Cr, 10%-30% Mn and 5% Mo. He found that the impact toughness decreased with increasing manganese content. At a manganese content of 22%, he observed a sharp drop in the impact energy. The reason for these differences in results can be ascribed to the fact that nickel and molybdenum were present in the alloys studied by Neef [19] and the manganese contents were very high (up to 30%). Tensile tests performed on the samples showed the effects of manganese and chromium on the yield strength, ultimate tensile strength, reduction in area (RA) and elongation values. The results of these tests are presented in Table 3. The effects of manganese and chromium on the ultimate tensile strengths of the alloys are shown in Fig. 14. From this figure, it can be seen that manganese addition increases the ultimate tensile strength of the alloys to levels of about 800 MPa in the

Table 2 Results of Charpy impact tests Alloy

Test 1 (J)

Test 2 (J)

Test 3 (J)

Average (J)

Normalized average (J cm -2)

5581 5591 5601 5611 5621 5631 5641 5651

10 18 30 33 6 7 37 26

11 26 25 37 5 6 32 33

20 23 31 32 7 8 27 32

14 22 29 34 6 7 32 30

17 28 36 43 8 9 40 38

Fig. 10. SEM Ii'actograph showing the extent of the brittle li'acture surface of the 17%Cr 3%Mn alloy at 35 x magnification.

M. Kemp et al. / Materials Science and Engineering A199 (1995) 183-194

Fig. 11. SEM fractograph showing the small ductile regions in the predominantly brittle fracture surface of the 1 7 % C r - 5 % M n alloy (magnification 200 × ).

17%Cr-9%Mn alloy. The increase in strength of these alloys is probably due to an increase in the amount of martensite in the alloy, which is either formed during cooling or as a result of the transformation of metastable austenite to martensite during testing, or a combination of both of these effects. This result is in agreement with the findings of Wright and Wood [20], who concluded that tensile and yield strengths increase, and ductility decreases, as the ferrite phase percentage in F e - 1 2 C r - 3 M n , ferrite-martensite microduplex alloys decreases. The decrease in ductility is a result of an increase in the phase percentage of martensite and a decrease in the phase percentage of ferrite.

Fig. 12. Transmission electron micrograph of a manganese sulphide inclusion (magnification 5000 x ).

191

Fig. 13. SEM fractograph showing decohesion between the grains after impact testing (magnification 90 x ).

An SEM examination of the fracture surface shows that delamination occurs in the tensile samples, which increases the strength of the alloys by retarding crack propagation. The extent of delamination increases with increasing manganese content, thereby also increasing the strength. As mentioned previously, the addition of manganese results in the formation of a dual-phase microstructure, which reduces the susceptibility to cracking by the presence of alternate phases, increasing the amount of energy required for crack propagation. 3.4. General corrosion results

The effects of manganese, heat treatment temperature and chromium content on the general corrosion properties of the alloys were examined in both reducing and oxidizing acids using anodic polarization scans. Fig. 15 shows the effect of an increase in manganese content on the critical current densities of the 17% and 20% chromium alloys tested in deaerated 0.1 N H 2 S O 4 at 25 °C. From this graph, it can be seen that manganese has a detrimental effect on the ability of these alloys to passivate, as the critical current density increases with increasing manganese addition for both chromium levels. The heat treatment temperature has little effect on the critical current density of the alloys tested in sulphuric acid. The addition of chromium, on the other hand, leads to a decrease in the critical current density and thus eases passivation of the alloy. An increase in the critical current density with increasing manganese content was also observed by Fourie and Bentley [21], Wilde and Armijo [22] and Miyahara et al. [23]. However, manganese additions have little effect on the passive current density and the passive range of the alloys examined. This is in agreement with the results obtained by Miyahara et al. [23], who conducted research into 12% chromium steels with manganese contents in the range 0%-30%. The effects of the manganese and chromium contents on the corrosion rates of the alloys tested in 0.1 N H 2 S O 4 at 25 °C are shown graphically in Fig. 16. As can

M. Kemp et al. / Materials Science and Engineering A 199 (1995) 183 194

192

Table 3 Effect of composition of alloy on mechanical properties Alloy

UTS1 (MPa)

UTS2 (MPa)

UTS3 (MPa)

Ave (MPa)

Elong. (%)

Elong. (%)

Elong. (%)

Ave (%)

RA1 (%)

RA2 (%)

RA3 (%)

Ave (%)

Yield (MPa)

Yield (MPa)

Yield (MPa)

Ave (MPa)

5581 5591 5601 5611 5621 5631 5641 5651

620 677 760 812 560 620 650 650

615 677 767 812 568 622 648 652

625 677 767 814 580 613 650 650

620 766 765 813 569 618 649 651

22 22 15 18 20 25 26 30

25 21 15 16 17 24 27 29

25 21 16 15 24 22 26 29

24 21 15 16 20 24 26 29

53 45 28 25 40 40 38 35

53 45 28 25 40 40 38 35

55 45 30 15 28 30 32 35

53 45 28 20 36 37 36 35

520 537 490 390 355 360 366 366

502 531 519 390 360 360 396 384

525 554 519 407 355 360 390 372

516 541 509 396 357 360 384 374

UTS, ultimate tensile strength; Elong., elongation; RA, reduction in area; Ave, average.

be seen from this graph, an increase in the manganese content results in an increase in the corrosion rates. This shows that manganese additions are detrimental to stainless steels from a general corrosion point of view. Potentiodynamic testing was also carried out in a 6.7 N acetic acid test solution to investigate the corrosion properties of the experimental alloys in oxidizing organic acids. The low corrosion rates (0.25 m m y e a r - t ) observed during these potentiodynamic tests showed little variance with increasing manganese content or with different heat treatment temperatures. The entire range of alloys performed well in this mildly oxidizing organic acid and the corrosion rates were comparable with AISI Type 304 stainless steel (0.17 m m y e a r - ~ ) and far superior to the 430 stainless steels (0.78 m m y e a r - ~). This indicates that the experimental alloys could perform satisfactorily when used under oxidizing conditions such as in brandy distilleries or food processing.

3.5. Pitting corrosion results The pitting corrosion properties of the alloys were examined using cyclic polarization sweeps at a scan rate of 1 mV s - ~ in a deaerated 0.025 N NaCI solution held at 25 °C.

Fig. 17 shows the effect of manganese on the pitting potentials of the 17% chromium alloys. F r o m this graph, it can be seen that an increase in the manganese content leads to a decrease in the pitting potentials of the alloys and thus decreases the resistance to pitting. This can be explained in terms of an increase in the amount of manganese oxide inclusions with an increase in the manganese content. The presence of increased amounts of manganese-rich inclusions with increasing manganese content was confirmed using SEM and EDS studies. The increase in the amount of manganese-rich inclusions creates more pit initiation sites in the alloys. A SEM investigation showed that pitting occurs more or less equally in both the ferrite and austenite/ martensite phases. However, pitting appears to occur preferentially along the grain boundaries between the ferrite and austenite/martensite phases. This is probably because more precipitates are present along the grain boundaries. Fig. 18 shows a graph of the repassivation potential as a function of the manganese content for the 17% chromium alloys. This graph shows that an increase in manganese addition leads to a decrease in the repassiA4000

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H2SO4.

M. Kemp et al. / Materials Science and Engineering A 199 (1995) 183-194 160

results are in agreement with those of Fourie and Bentley [21] and Degerbeck and Wold [24], who found that an increase in manganese addition results in a decrease in the pitting potentials of manganese alloyed stainless steels.

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F r o m the experimental work carried out, a number of conclusions can be drawn with regard to the effects of manganese and heat treatment temperature on the corrosion and mechanical properties of 17% and 20% chromium stainless steels. (1) Hardness testing shows that an initial heat treatment at 1050 °C, followed by a heat treatment at 600 °C for 1 h and water quenching, results in alloys in their softest possible condition. (2) Image analysis reveals that, under the conditions of this investigation, manganese acts as an austenitestabilizing element. (3) Tensile testing shows that an increase in the manganese content results in an increase in the ultimate tensile strength while the yield strength remains virtually unaffected. (4) Impact toughness testing of the alloys shows that an increase in impact toughness occurs as a result of an increase in manganese addition. (5) Accelerated electrochemical tests performed in a 0.1 N H2SO 4 solution show that an increase in manganese content leads to an increase in the corrosion rate and the critical current densities of these alloys. (6) Results of electrochemical tests show that manganese additions have virtually no effect on the passive ranges of the alloys tested in both oxidizing and reducing acid solutions. (7) After an oxalic acid etch test, reduced precipitation is observed with increasing manganese content. (8) The pitting potentials of the alloys decrease when tested in 0.025 N NaC1 as the manganese content is increased. (9) Manganese additions have a slightly detrimental effect on the corrosion properties of the alloys tested in a 6.7 N acetic acid solution. (10) A distinct reduction in the ability of these alloys to passivate is noted in the reducing acid environment as the manganese content is increased. (11) A similar, but not as pronounced, effect is noted when these alloys are tested in an oxidizing solution (6.7 N acetic acid).

Fig. 18. G r a p h showing the effects of manganese on the repassivation potentials of the 17%Cr alloys tested in 0.025 N NaCI. Acknowledgements

vation potential of the stainless steel, once again confirming the detrimental effect of manganese on the pitting corrosion resistances of these stainless steels. These

The authors wish to thank the University of the Witwatersrand for permission to publish this paper and

194

M. Kemp et al. / Materials Science and Engineering A 199 (1995) 183 194

Dr. M. Witcomb for his help with the transmission electron microscopy work. We also wish to extend our appreciation to Columbus Stainless (Pty) Ltd. for producing the alloys and for financial support, and to Mintek and the Scientific Investigation Bureau for the use of equipment.

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[11] E. Folkhard, Welding Metallurgy of Stainless Steels, SpringerVerlag, Vienna, 1988, p. 29. [12] C.M. Hsaio and E.J. Dulis, Phase relationships in C r - M n - C - N stainless steels, Trans. Am. Soc~ Met., 51 (1959). [13] J.S. Lee, E.E. Stansbury and S.J. Pawel, Determination of pitting susceptibility using a new sample preparation technique, Corrosion, 45 (2) (1989) 134 136. [14] F.C. Hull, Delta ferrite and martensite formation in stainless steel, Welding J., 52 (1973). [15] E.R. Szumachowski and D.J. Kotecki, Effect of manganese on stainless steel weld metal ferrite, Welding J., 63 (1984). [16] A.M. Ritter, M.F. Henry and W.F. Savage, High temperature phase chemistries and solidification mode predictions in nitrogen-strengthened austenitic stainless steels, Metall. Trans. A, 15 (1984). [17] J. Hochmann, The role of manganese additions in austenitic stainless steels, Mater. Technol., December (1977). [18] T. Kato, S. Fukiu, M. Fujkura and K. Ishida, Structural stability and mechanical properties of Fe Mn Cr alloys, Trans. Iron Steel Inst. J, 16 (1976). [19] H.J.C. Neef, Austenitic manganese alloyed stainless steel: the AISA Series Part 2, Roesvat Staal, No. 3, June, 1989. [20] R.N. Wright and J.R. Wood, Fe C r - M n microduplex ferritic martensitic stainless steels, Metall. Trans. A, 8 (1977) 2007-2011. [21] J.W. Fourie and A.P. Bentley, The effect of variations in alloying content on the corrosion resistance of F e - C r Mn stainless steels, Proc. Conf. on Manganese Containing Stainless Steels', Cincinnati, OH, 10-15 October, 1987. [22] B.E. Wilde and J.S. Armijo, Influence of Si and Mn on corrosion behaviour of austenitic stainless steels, Corrosion, 24 (1968). [23] K. Miyahara, R. Sugihara, T. Satch and Y. Hosoi, Effects of alloying elements of Mn, C and N and ageing treatments on corrosion resistance of F e - C r - M n alloys, Proe. Int. Conf. on Stainless Steels, Chiba, Iron and Steel Institute of Japan, 199I, p. 139. [24] J. Degerbeck and E. Wold, Werkst. Korros., 25 (1974) 172. [25] G. Kirchner and B. Uhrenius, Acta Metall., 22 (1974).