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JOURNAL OF IRON AND WEXL RESEARCH, I"ATI0NAL.
2010, 17(8): 50-54
Influence of Tempering Process on Mechanical Properties of 00Cr13Ni4Mo Supermartensitic Stainless Steel ZOU De-ningl ,
(1. School of Metallurgy and Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, Shaanxi, China; 2. State Key Laboratory of Advanced Stainless Steel Materials, Taiyuan 030003, Shanxi, China)
Abstract t To investigate the influence of tempering process on microstructural evolutions and mechanical properties of 00Cr13Ni4Mo supermartensitic stainless steel (SMSS), specimens were tempered in the temperature range of 520720 C . for 3 h followed by air cooling and an optimized tempering temperature was chosen to prolong holding time from 3 to 12 h. After heat treatments, microstructure examination was conducted by scanning electron microscope, X-ray diffraction examinations, hardness measurements and tensile tests. The results revealed that the superior mechanical properties were achieved by quenching at 1040 'C for 1 h+water cooling and tempering at 600 'C for 3 h i air cooling. Increasing isothermal tempering time could improve the toughness notably. It was believed that the property was correlated with the microstructure of tempered lath martensite and retained austenite. More retained austenite content is beneficial to the higher toughness of the SMSS. Key words: supermartensitic stainless steel; tempering; mechanical property; retained austenite
With more active exploitation of deep and ultradeep wells in the oil/gas industries, the martensitic stainless steel API-13Cr often suffers from poor resistance to corrosion and failure in the surrounding containing H2S, CO, and C1- , which is hard to satisfy the working requirements. Therefore, supermartensitic stainless steel (SMSS) has been increasingly used as pipelines applications since the late 1 9 9 0 ~ " - ~and ~ has the tendency to replace duplex stainless steel in stripper wells owing to its excellent combination of toughness, weldability , corrosion resistance, ease of heat treatment, and comparatively low price. With the unique steelmaking technologies, the steel has improved the conventional martensitic stainless steel by decreasing carbon content (
ture, owing to the existence of amounts of finely distributed austenite along the martensite interlath boundaries and prior austenite grain boundariesCs1. The retained austenite formed during the heating process and retained to room temperature is related to the additional alloying elements (Ni and Mo)[~'. Inappropriate heat treatment may cause important changes in microstructure and serious decrease in mechanical property value.^[^-^^. Since the superior performance of these steels is strongly dependent on a quenching and tempering type of thermal history and chemical composition balance, a proper tempering for the steel with a certain chemical composition is necessary to be studied to achieve microstructure stability. In the present study, the detailed results are presented to investigate the influence of tempering parameters on microstructures and properties of the new developed 00Cr13Ni4Mo SMSS. The principal objective is to achieve the excellent combination of toughness and strength for pipeline steel used in the oil/gas industries.
Sponsored by Special Project of Shaanxi Education Department of China (07JK309) ; Xi'an University of Architecture and Technology of China (JC0714) E-mail: zoudeningBsina. com; Received Date: July 14, 2009 Biography:ZOU Dening(1964-), Female, Doctor, Professor;
hot forged to an intermediary square billet with section area of 80 mmX 80 mm and hot rolled to round bars of 25 mm in diameter. The hot working temperature was 1150 "C for forging and rolling. The chemical composition of the steel is given in Table 1.
1 Experimental The steel was produced in a laboratory vacuum induction-melting furnace. 100 kg square cast ingots with dimensions of 150 mmX 150 mmX 500 mm were
(mass percent, %)
Table 1 Chemical composition of the SMSS C
The specimens were subjected to solution treatment at 1040 "C for 1 h followed by water cooling to room temperature. 1040 'C was sufficient to transform the steel fully into austenite and to form martensite phase upon quenching. After that, the solution treated specimens were tempered at 520, 560, 600, 640, 680 and 720 "C for 3 h , respectively, followed by air cooling and the proper tempering temperature was chosen according to the test results. Finally, the solution treated specimens were tempered at the chosen temperature for 3, 6, 8 , 10, and 12 h , respectively, followed by air cooling. Mechanical properties were evaluated by tensile and hardness tests at room temperature. The tensile tests were performed in accordance with GB/T 2282002. The hardness of specimens was measured by Rsckwell (Vickers) hardometer. Each value was the average of hardness values at three points. T o characterize the microstructures, the samples were subjected to the standard grinding and polishing techniques before etching with aqua regia ( a mixture of 75% of HC1 and 25% of HNO, 1. Properly etched samples were examined by scanning electron microscope (SEMI ( Model: Leo438VP) using secondary electron emission at 15 to 20 kV. The content of retained austenite in the steel was quantitatively measured by comparing the integrated Co Xray diffraction intensities of the ferrite and austenite phases with the theoretical intensities.
Influence of Tempering Process on Mechanical Properties of Supermartensitic Stainless Steel
Results and Discussion
Fig. 1 (a) shows the variation of ultimate tensile strength, yield strength (RN.) , and elongation as a function of different tempering temperatures for 3 h. It can be seen that with increasing tempering temperature from 520 to 560 ' C , the ultimate tensile strength and yield strength sharply decrease, and the elongation obviously increases. From 560 to 600 'C , the ultimate tensile strength and yield strength still
- 18 . 16
600 660 TemperaWC
Variation of tensile properties and hardness with tempering temperature
decrease and the former drops more slowly, and the elongation increases to a maximum value at 600 %. From 600 to 680 'C , there is a slight increase in the ultimate tensile strength and yield strength, and a sharp decrease in the elongation. After 680 *C , the ultimate tensile strength and yield strength rises significantly, and elongation decreases more sharply. Fig. 1 (b) shows the influence of various tempering temperatures on the hardness values. It is found that the average HRC hardness is enhanced from 28 to 32 with an increase in tempering temperature, but the variation is unconspicuous. A maximum value of hardness is achieved when tempering at 680 *C for 3 h, and tempering between 680 *C to 720 C , the hardness remains a constant. Fig. 2 shows the variation of elongation and hardness with holding time when tempering at 600 %. It can be seen that the elongation obviously increases from 18% to 21% with the extension of holding time. However, the time has little effect. on hardness values. Only in the first interval from 3 h to 6 h,
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18 . *
mains approximately constant after holding more than 6 h. T o explain the above variation trends, heattreated specimens were analyzed by scanning electron microscope and X-ray diffraction instrument. Fig. 3 shows the microstructures of samples tempering a t different temperatures for 3 h. T h e low carbon tempered martensite exhibiting lath morphology
(a) 560 'C; (b) 600 *C; (c) 640 *C; (d) 720 %. Microstructures of samples tempered at different temperatures for 3 h
pering from 600 to 720 'C , the width of the laths is found to be coarsening, which is owing to an obvious decrease in the retained austenite content. Fig. 4 shows the microstructures of samples tempered at 600 "Cfor 6 h and 1 2 h , respectively. The refined complex structure of lath martensite and retained austenite ( R A ) is also found. With increasing the holding time, the retained austenitic grains are elongated and the percent of retained austenite is enhanced. It is well known that this low carbon tempered martensite is responsible for the high tensile strength and hardness of the SMSS. Tempering from 520 to 720 ' C , the quenched martensite be-
comes softening, which leads to the elimination of internal stress, the decrease in the dislocation density, and the occurrence of retained a u ~ t e n i t e ~ ~ *This ~-'~~. retained austenite exhibiting fine membraniform distribution between the martensite laths in the microstructure has a good effect on the toughness of the steel. T h e toughening mechanism of the austenite particles is associated with transformation-induced plasticity (TRIP)C51. T h e content of retained austenite at different tempering temperatures is shown in Fig. 5 (a). It is clear that retained austenite gradually increases as tempering temperature changes from 520 to 600 'C ;
Influence of Tempering Process on Mechanical Properties of Supermartensitic Stainless Steel
(b) Holding for 12 h.
(a) Holding for 6 hi
Microstructures of samples tempered at 600 "c @>
TemperatureE (a) Tempering at different temperatures for 3 h;
6 8 Holding timeh
(b) Tempering at 600 'C for different time.
Variation curves of retained austenite
after that, the content of retained austenite decreases with temperature increasing up to 720 "C. Around 600 "C , the tempered steel has the maximum amount of austenite up to 11. 5% [Fig. 3 (b)]. This is attributed to the content of reversed austenite (tempered martensite-austenite) and its stability during the process of tempering["'. According to Leem' s research resultC"] , the amount of reversed austenite increased with increasing the tempering temperature. Hence, a t higher tempering temperature from 600 to 720 'C , a large amount of reversed austenite may be formed in the steel. However, with increasing the content of reversed austenite, the concentrations of austenite stabilizing elements such as nickel decrease gradually, indicating that the stability of reversed austenite becomes lower with increasing the tempering temperature owing to a rise in M , temperature. Another reason for the decrease in the stability of reversed austenite is the increased concentration of quenched-in Consequently, much reversed austenite that formed a t these temperatures
retransforms t o fresh martensite during cooling above 600 'C. Owing to the lower amount of reversed austenite and its high stability at lower tempering temperature from 520 to 600 'C, only a few retained austenite precipitates in the steels tempered below 600 %. Fig. 5 ( b ) shows the relationship of retained austenite content and holding time for tempering at 600 "C. T h e content of retained austenite becomes more with an increase in the holding time for tempering, and it is found to reach about 1 4 . 1 % after holding for 12 h. This is owing to the presence of the transformation of martensite to austenite and the growth of austenite grains. Several studies have been reported that the carbide precipitation' at different tempering temperatures can strengthen the martensitic stainless steel, for example, M,,C, gradually forms and grows with increasing tempering temperature from 500 'C to the For the steel studied here, AC1 when tempering from 520 to 600 'C , carbide precipitates causing a small amount of secondary hardening
Journal of Iron and Steel Research, International
may offset or partially offset the softening effect of the reversed austenite. However, specific analysis for this aspect was not conducted. In this work, it is noticeable that the variation of retained austenite is in correspondence with the elongation change when increasing the tempering temperature for 3 h or tempering at 600 'C for various holding time. The reason for the variation of tensile properties with tempering temperature changes can be described as follows: 1) Tempering from 520 to 560 "2, a decrease in the dislocation density and an increase in retained austenite are responsible for the decrease in tensile strength and the increase in elongation. 2) Tempering from 560 to 600 'C , the generation of carbide and secondary hardening may result in slight descending of ultimate tensile strength. 3) Tempering from 600 to 680 "C , with a decrease in the amount of retained austenite, the elongation decreases and the tensile strength increases gradually. 4) Tempering in the higher temperature range from 680 to 720 "C, a number of reversed austenite becomes less stable and transforms into fresh martensite during cooling to room temperature, which causes the ultimate strength to increase sharply and also causes rapid decrease in the elongation. The effects of tempering temperature on the hardness of the steel are not obvious. The values of hardness increase feebly with increasing tempering temperature from 520 to 680 'C , after which a maximum value occurs. This may be due to the precipitation of carbide and more and more partial transformation of reversed austenite to fresh martensite. In addition, as holding time goes on, the toughness of the steel increases obviously owing to the increase in retained austenite content. The elongated austenite grains and the precipitation of carbonitride during tempering and holding at 600 'C may be the primary cause of slight hardness values variation.
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