Grain growth in austenitic stainless steels

Metallography

349

Grain Growth

in Austenitic Stainless Steels*

J. K. STANLEY+ The Aerospace

AND

Corporation,

A. J. PERROTTA El Segundo,

California

Despite the extensive use of Type 347 stainless-steel tubing for regeneratively cooled thrust chambers, very little is known about its grain growth characteristics. Lacking, also, is information on the other 18 Cr-8 Ni austenitic steels. Therefore, grain growth curves for AISI Type 304, 316, and 321 were obtained and compared.

Grain growth characteristics were established at temperatures between 1900°F and 2300”F, for periods of 5, 15, and 60 minutes. Rapid heating and cooling were used with all samples to simulate, to some degree, conditions experienced in thrust chambers. Some similarity in grain growth behavior of Types 321 and 347 was noted; rapid grain growth was found to occur at about 2000°F. Some grain growth similarity also exists between Types 304 and 316; grain growth was rapid at temperatures above 1900°F. Where grain growth occurred, the greatest changes were during the first 15 minutes at temperature. Where micrographs are available in a failure analysis, these data can be used to make estimates of the temperatures to which the austenitic steel tubes were heated.

Introduction Data on the grain growth of stainless-steel tubes This

are virtually nonexistent

situation also applies to grain growth in other fabricated metal forms,

such as sheet, plate, bar, wire, and extrusions. The grain growth characteristics (that is, changes in grain size with temperature) of these forms may differ, even though the starting material came from the same heat ingot. Ordinarily, one is not concerned with the change in grain size with temperature. As high strength levels are associated with fine grain sizes, it is almost axiomatic that the steel producer always strives for as fine a grain size in stainless steels as conventional processing allows. The stainless steels in their various forms are used in air from ambient to about 1600”to 1800°F. At these temperatures, the * This work was supported by the U. S. Air Force under Contract F04701-68-C-0200. + Manager, Applied Metallurgy Section, Materials Sciences Laboratory, The Aerospace Corporation. Metallography, Copyright

0

2 (1969) 349-362

1969 by American Elsevier Publishing Company, Inc.

350

J. K. Stanley and A. J. Perrotta

grain size is still relatively small, although long holding times around 1800°F tend to coarsen the grain structure slightly. It is a well-known engineering fact that, as the temperature

of all metals is

raised, the grains grow by a phenomenon known as coalescence-that is, the consuming of grains by cannibalistic grains that are thereby increased in size. This phenomenon Being

is ordinarily called grain growth.

a surface tension

process,

grain growth is particularly

sensitive

to

temperature, although other factors are operative, such as time, composition, level of impurities (for example, carbon, nitrogen), and inclusions (oxides and sulfides). With a given metal, the main parameters influencing grain growth are temperature

and time.

Despite the dependence of grain growth on many factors, it is surprising that the changes in grain size from lot to lot of stainless-steel tubing are not particularly large. Most of the grades examined-American

Iron and Steel Institute

(AISI)

coarsening temperature,

types-appear

to have their own characteristic

but none are really too different. Experience has shown that change in grain size within each AISI type from lot to lot ordinarily involves little noticeable variation in grain size for materials It is a pleasant

commentary

held for the same times at temperature.

on austenitic

stainless-steel

producers

who so

carefully tailor and produce commercial products of such uniform quality. This is fortunate, as it allows one to estimate tube temperatures within a temperature

range, regardless of the source of the stainless tubing.

Experimental Four

Procedure

austenitic

stainless

steels,

in the form of tubing,

were used in the

experiments. The stainless-steel tubes measured 0.250-inch o.d. by 0.625-inch wall thickness. Actual analyses of the specimens are given in Table I. The specimens

before heat-treatment

were cut to approximately

0.25-inch

lengths and were cleaned by dipping into 50/50 nitric acid. The experiments TABLE

I

ACTUAL COMPOSITIONS(PERCENTAGE) Material

C

Mn

Si

Cr

AISI-304 AISI-316 AISI-321 AISI-347

0.07 0.04 0.06 0.07

1.40 1.77 1.47 1.43

0.60 0.21 0.67 0.62

19.12 17.85 17.76 17.66

MO

2.99

Fe Bal Bal Bal Bal

Ti

Cb

0.43 0.74

Ni 10.43 13.13 10.88 12.07

Amtenitic Stainless Steels were conducted

351

by heating the samples rapidly to temperatures

(pulled into

the hot zone) ranging from 1900” to 2300°F for periods of 5, 15, and 60 minutes in a hydrogen atmosphere. A few experiments were also run in argon. More specifically, specimens of each material were tied to a Nichrome wire. Sets of five were pulled into the hot zone of a tube furnace at a given temperature and were held for a given time. Then they were pulled from the heat zone and cooled to room temperature in hydrogen. This rapid heating and cooling method was used to simulate, somewhat, conditions existing in a thurst chamber. The annealed specimens were then prepared for metallographic using standard techniques.

examination,

The steels were etched with 4 parts HCl plus 1 part

HN6,.

0.14

I

I

I

5

I

I

I

30

I

I

I

I

45

TIME (min) 1.

Grain growth in AISI Type 304 stainless steel.

I

60

352

J. K. Stanley and A. J. Perrotta

Photomicrographs

of each specimen were taken on Polaroid film, and mea-

surements were made directly on the print. photographed on fine-grain film.

Typical

grain sizes were also

The intercept (Heyn) method was used to determine the grain sizes of the metals. (The method is described in ASTM, E-112-61, published by the American Society of Testing Materials, Philadelphia.) One or more straight lines in both the longitudinal (nl) and transverse (q) directions were drawn on the photomicrograph

to yield at least 50 intercepts. In cases where a variable TABLE AISI304

II

STAINLFSS

STEEL

ASTM micrograin size number

Diameter of average grain (mm)

Average intercept distance (mm)

As received

8.0

0.023

0.020

1900°F 5 min 15 min 60 min

7.0 6.0 5.5

0.032 0.044 0.056

0.029 0.039 0.050

2000°F 5 min 15 min 60 min

5.5 5.0 4.0

0.050 0.067 0.087

0.045 0.062 0.078

2100°F 5 min 15 min 60 min

5.5 4.5 3.5

0.055 0.080 0.094

0.049 0.071 0.086

2200°F 5 min 15 min 60 min

4.0 3.5 3.5

0.087 0.094 0.111

0.078 0.086 0.108

2250°F 5 min 15 min 60 min

3.5 3.5 3.0

0.092 0.098 0.124

0.084 0.092 0.112

2300°F 5 min 15 min 60 min

3.5 3.5 2.5

0.100 0.111 0.148

0.094 0.109 0.130

Test condition

353

Austenitic Stainless Steels grain size was evident, average.

If the grains

as many as 200 intercepts were very coarse,

derive an average. The length of the lines, divided is used to obtain intercept

length

the average

length

of grains

(NJ

intercepted

of the grains.

a good

were used to by them,

The

average

is

NI, = ASTM

to obtain

photomicrographs

by the number

intercept

111

The

were counted

several

grain

+

i No. of grains

nt No. of grains

>

2

size number,

which

is familiar

to most

metallurgists,

can be

0.13-

0.1 I -

-

TIME FIG.

2.

Grain

growth

in AISI

(min) Type

316 stainless

steel.

354

J. K. Stanley and A. J. Perrotta

determined from tables if the intercept known (refer to ASTM, E-112-61).

length or average grain diameter is

The grain size of starting material was as follows: Material

Grain Diameter (mm)

AISI Type 304

0.023

AISI Type 316

0.016

AISI Type 321

0.011

AISI Type 347

0.012

TABLE

III

AISI 316 STAINLFSSSTEEL ASTM micrograin size number

Diameter of average grain (mm)

Average intercept distance (mm)

As received

9.0

0.016

0.013

1900°F 5 min 15 min 60 min

7.5 6.5 6.0

0.025 0.039 0.044

0.022 0.034 0.039

2000°F 5 min 15 min 60 min

7.0 6.0 5.5

0.031 0.047 0.050

0.028 0.042 0.044

2100°F 5 min 15 min 60 min

6.0 5.5 4.5

0.040 0.053 0.072

0.036 0.048 0.064

2200°F 5 min 15 min 60 min

5.5 5.0 4.5

0.053 0.065 0.079

0.047 0.058 0.073

2250°F 5 min 15 min 60 min

5.0 4.5 3.5

0.058 0.080 0.098

0.052 0.073 0.091

2300°F 5 min 15 min 60 min

4.5 3.5 3.0

0.075 0.094 0.138

0.067 0.086 0.123

Test condition

Austenitic Stainless Steels

355

3: I

I

45

I

_

60

TIME (minl FIG. 3.

Grain growth in AISI

Type 321 stainless steel.

Results The data recorded as time-temperature

curves are given in Fig. 1 (Type 304),

Fig. 2 (Type 316), Fig. 3 (Type 321) and Fig. 4 (Type 347). The grain growth curves for Type 304 steel (Table II) are shown in Fig. 1. At none of the times do the grains reach an equilibrium grain size. Growth was rapid at 2100°F

and higher.

Similar curves were obtained for Type 316 steel. (See Fig. 2 and Table III.) At 1900°F and 2000”F, grain growth was very rapid and was continuing even at the end of 60 minutes.

356

J. K. Stanley and A. J. Pmotta TABLE AISI 321

IV

STAINLFSS

STEEL

ASTM microgram size number

Diameter of average grain (mm)

Average intercept distance (mm)

10.0

0.011

0.010

1900°F 5 min 15 min 60 min

9.5 8.5 8.0

0.014 0.019 0.022

0.012 0.017 0.019

2000°F 5 min 15 min 60 min

7.0 5.0 4.5

0.031 0.063 0.074

0.027 0.056 0.066

2100°F 5 min 15 min 60 min

6.0 4.5 3.5

0.047 0.073 0.094

0.042 0.065 0.086

2200°F 5 min 15 min 60 min

4.5 3.5 3.5

0.077 0.092 0.095

0.069 0.083 0.087

2250°F 5 min 15 min 60 min

4.0 3.5 3.5

0.084 0.093 0.098

0.076 0.084 0.091

2300°F 5 min 15 min 60 min

3.5 3.5 3.0

0.093 0.106 0.130

0.084 0.098 0.115

Test condition As received

Grain size change in Type 321 (titanium-stabilized)

shows significant variation

at 2000°F and higher. (See Fig. 3 and Table IV.) Grain size changes in Type 347 (columbium stabilized)

were significant

at

temperatures above 2000°F. (See Fig. 4 and Table V.) Owing to the possibility that hydrogen might have some effect on the grain growth because it is a reducing atmosphere specimens,

a few experiments

and could possibly decarburize

the

were conducted in purified argon using Type 347

steel. A series of runs was made at 2250°F.

This set of runs (Fig. 4) coincided

Austenitic Stainless Steels

357

TIME (min)

FIG. 4.

Grain growth in AISI Type 347 stainless steel.

with the hydrogen series. The grain sizes found were within experimental It was concluded

that the use of hydrogen

does not introduce

anomalies

error. into

the measurements.

Discussion

of Results

Differences in grain growth between the four austenitic steels were noted. Grain coarsening is noticeable at 1900°F in Types 304 and 316; grain growth occurs at about 2000°F with Types 321 and 347.

358

J. K. Stanley TABLE AISI

and A. J. Perrotta

V

347 STAINLE~S~TEEL

ASTM

Diameter

of

Average

Test

micrograin

average

intercept

condition

size number

grain (mm)

distance (mm)

10.0

0.012

0.010

As received 1900°F 5 min

9.5

0.013

0.011

15 min

9.5

0.014

0.012

60 min

9.0

0.016

0.014

2000°F 5 min

9.0

0.016

0.014

15 min

8.5

0.018

0.016

60 min

7.0

0.034

0.030

0.027

2100°F 5 min

7.0

0.030

15 min

6.0

0.040

0.036

60 min

5.5

0.054

0.048

0.043

2200°F 5 min

6.0

0.048

15 min

5.0

0.067

0.060

60 min

4.5

0.076

0.068

2250°F 5 min

5.5

0.056

0.049

15 min

4.5

0.070

0.062

60 min

3.5

0.094

0.086

2300°F 5 min

4.5

0.078

0.069

15 min

3.5

0.096

0.089

60 min

2.5

0.156

0.140

Because numbers are difficult to visualize, several sets of photomicrographs were also made to illustrate the grain growth process. Figure 5 shows the change in grain size of Type 304; Fig. 6 of Type 316; Fig. 7 of Type 321; and Fig. 8 of Type 347. These grain growth data can be used for estimating the temperatures to which

Austenitic Stainless Steels

Type

347 stainless-steel

hardware

(gas coolers

359

regeneratively

is seen in micrographs of a failure cluded that the temperature must temperature Greater

thurst

chamber

have been heated.

analysis involving Type have been over 2000°F

tubing

If a coarse

are made.

If a grain

firing time is known

(with

size measurement it generally

and other grain size

347, it can be conbecause this is the

at which significant grain coarsening occurs. accuracy of temperature estimation can be achieved

size measurements accumulated

cooled

and superheaters)

if actual

is available

is to fractions

grain

and the

of a second),

360

J. K. Stanley and A. J. Perrotta

then the curves shown can be used to estimate the temperature

experienced

by

them. Assume, for instance, that average grain diameter of 0.07 mm is determined on a failed Type 347 tube. Assume that the thrust chamber had been fired for 200 seconds or 3.3 minutes. Referring to Fig. 4, it is easy to pick off 3.3 minutes and 0.07 mm, which are found to correspond

to a tube temperature

of about

2300°F. Obviously, such information gives estimates only; no great accuracy is claimed because of the unknowns, such as actual time at temperature variation itself.

Austenitic Stainless Steels

361

Summary Grain growth characteristics

of four austenitic stainless-steel

347, and 316) were established at temperatures

(Types 304, 321,

between 1900°F and 2300°F

for

periods of 5, 15, and 60 minutes. Rapid heating and cooling was used with all samples to simulate conditions experienced in thrust chambers. Experiments were conducted in hydrogen atmosphere. Some similarity in grain growth behavior of Types 321 and 347 was noted;

362

J. K. Stanley

rapid grain growth occurs at about 2000°F.

and A. J. Perrotta

Some grain growth behavior also

exists between Types 304 and 316; grain growth is rapid at temperatures above 1900°F. The greatest changes in grain size occur during the first 15 minutes at temperature. These data can be used in failure analysis to make estimates of temperatures to which Type 347 was heated in thrust chambers, Accepted September

18, 1969

coolers, and superheaters.