The fundamental factors influencing the behaviour of welded structures: The effect of subcritical heat treatment on the transition temperature of a low carbon ship plate steel

on

THE FUNDAMENTAL FACTORS INFLUENCING

THE BEHAVIOR OF WELDED STRUCTURES:

The Effect of Subcritical Heat Treatment on the Transition Temperature of a

Low Carbon Ship Plate Steel

by

E. B. Evans and D. J. Garibotfi

CASE INSTITUTE OF TECHNOLOGY

Transmitted through NATIONAL RESEARCH COUNCIL'S

COMMITTEE ON SHIP STEEL Advisory to

SHIP STRUCTURE COMMITTEE

tAEORATOPJUM VOOR

riL±I

L)lN.)I

Division of Engineering and Industrial Research

National Academy of Sciences National Research Council

Washington, D. C. October30, 1953 SERIAL NO. SSC.61

LAEORATOJUM VOOR

0096

Fifth PROGRESS REPORT (Prolect SR-99)

SHIP STRUCTURE COMMITTEE

October 30, 193

Dear Sir:

As part of its research program related to the

improvement of hull structures of snips, the Ship

Struc-ture Committee is sponsoring an investigation on "The

Fundamental Factors Influencing the Behavior of Welded Structures under Conditions of Multiaxial Stress and Variations of Temperature" at the Case Institute of

Technology. Herewith is a copy of the Fifth Progress

Report, SSC-61, of the investigation, entitled "The Fundamental Factors Influencing the Behavior of Welded

Structures: The Effect of Subcritical Heat Treatment

on the Transition Temperature of a Low Carbon Ship Plate

Steel" by E. B. Evans and D. J. Garibotti.

The project is being conducted with the advisory assistance of the Committee on Ship Steel of the National Academy of Sciences-National Research Council.

Any questions, comments, criticism _{or other matters}

pertaining to the Report should be addressed to _the

Secre-tary, Ship Structure Committee.

This Report is being distributed to those _individuals

and agencies associated with and interested in the work of

the Ship Structure Committee.

Yours sincerely,

K. K. COWART

Rear Admiral, U. S. Coast Guard

Chairman, Ship Structure Committee

MEMBER AGENCIES: _{ADDRESS CORRESPONDENCE TO:}

BUREAU OF SHIPS. DEPT. OF NAVY SECRETARY

MILITARY SEA TRANSPORTATION SERVICE. Dipl. OF NAVY SHIP STRUCTURE COMMITTEE

UNITED STATES CO.ST GUARO. TREASURY DEPT. U. S. COAST GUARO HEADQUARTERS

MARITIME ADMINISTRATION. DEPT. OF COMMERCI WASHINGTON 25. D. C.

FI FTH

Progress Report (Project SR-99)

The Fundarnenal Factors Influencing the Behavior of' Welded Structures: The Effect of' Subcritical Heat Treatment

on the Transition Temperature of a Low Carbon Ship Plate Steel

by

E. B, Evans D JG Garibotti

CASE INSTITUTE OF TECHNOLOGY

uride r

Department of the Navy Bureau of Ships NOBS--i-51+7O

BuShips Project No0 _{NS_011_076?}

for

TAELE OF CONTENTE

i

Table of' Contents . . o 3

i

L Ist of Ftiures 11 kbstract iii Introuuction e e a o e t

e tOCO

i

iaterial t Q o a o o o s s o o o o e e s o o o o 2 Proceciure e s o o o o e o s e e e e o o t o e o 3 Results Hardness Tests s o o o e e o e s o o e o e e 1+ Impact Tests o s s e e e o e o o o e O Ioicrostructtres e o e e 0 0 0 0 5 0 0 5 0 0 11+ DJ_SCU.SS1.Ofl© s o o s o e e o s e e s o e o o o e e e o 15' Corìclusion.s . , Future 1iork . t e t t t o e o o s t o i e o o o o o e e o e s e o o 23 21+ Bibliography . . e Q e o o 25'

LIST OF FIGURES

Title Page

l Effect of Various Aging Times and Temperatures on

the Hardness of "C" Steel after Water Quenching from 13000F o a t e a e e e * o o s a e o o o

2 Charpy V-Notch Transition Curves for As-Received

urn - -î

O O a e Q o o Q Q Q Q O Q O e o o o

3 Charpy V-Notch Transition Curves for "C" Steel Suberiticaily heated at 1300°F for

₁₅

Minutes and Air Cooled0 t O O Q O Q O *

+. _{Charpy V-Notched Transition Curves for "C" Steel}

SubcriticaJjy heated at 1300°F for

₁₅

Minutes and Water Quenched0 Aged at Roo

Temperature for Times Indicated G Q 11

Charpy V-Notched Transition Curves for "C" Steel Subcriticafly Heated at 1300°F for 15

Minutes and Water Quenched0 Aged at 125°F

for Times Indicated . . . 12

Charpy V-Notched Transition Curves for 1tC" Steel Subcritically Heated at 1300°F for

₁₅

Minutes and kater Quenched0 . . 12

7 Charpy V-Notched Transition Curves for "C" Steel Subcritically heated at 13000F for

₁₅

Minutes and Water Quenched0 Aged at 600°F

for Times Indicated . . . O O G O t O 0 13

Summary Curves Shoving Effect of Various Aging

Temperatures and Times on the Transition

Temperature and Hardness of "C" Steel0

Specimens Water Quenched from 13000F before

Aging a o o o o e o o a o o a o o o e e ₁₃

l4icrostructures of As-Received and Various

Quench-Aged Conditions . . 16

Effect of Room Temperature Aging Time on the

Transition Temperature and Hardness of "C"

Steel after Water Quenching from Temperatures

Indicated . . . . 18

11 Solution and Room Temperature Aging Effects on the Hardness and Impact Properties of "C"

Steel . . . , . . . 20

ii

5.

9

AB STRACT

An investigation was made to determine the impact transi-tion temperature and hardness changes attndant to the

quench-aging of Project Steel "C" a semi-killed ship plate steel1

Aging temperatures xtended over the range from 35 to 1100°F

after water quenching from 1300°F0

Both impact and hardness tests revealed that this steel can be severely embrittled by the quench-aging mechanism0 With aging temperatures up to 350°F, characteristic aging curves were obtained, i.e, the peak embrittlement and the time to attain this peak d&reased with increasing aging temperature0 For room temperature aging this peak amounted to a 900F increase

in transition temperature and

₂₅

points increase in Rockwell B

hardness above that of a series air cooled from 1300°F

(unembrit-tied condition)0 Specimens aged above

_350°F

overaged so rap-idly that no peak in the aging curve could be detected0

Metallographic examination of quench-aged specimens at 2000X showed that a two-stage precipitation reaction was

operative0 At low aging temperatures the precipitate was detected as a mottling of the ferrite grains; at higher aging

temperatures, where an overaged condition was rapidly reached,

the precipitate had grown so as to be resolvable0

It is believed that the quench-aging phenomenon is

re-sponsible for the ittle zone previouslfound in the

sub-critically heated region in weidments of this and similar ship

plate steels. This study suggests that a low temperature post

heat at 65O°F (the solution temperature below which quench-aging effects are absent) would lead to rapid overquench-aging in the brittle zone of ship plate weidments and thus largely eliminate

the embrittlement

INTRODUCTION

This Is the fifth and last progress report on the study of the zone of brittleness located adjacent to welds In

semi-killed steel plates0 The project has been sponsored by the Ship Structure Committee under Department of the Navy, Bureau

of Ships Contract Nob +5+7O and covers the period from

September 1, 1952,

to December

31, 1952e

The earlier report

on this phase of the investIgatIon covéred the work from

January 1, 1950, to September 1,

₁₉₅₂

Three Progress

Re-ports, SSC_21+(2), SSC-3i-3 and SSC-5+ summarized the

work on eccentric notch tensile testing of ship plate weld

ments under the same contract over the period from July 1,

l9+7,

to January 1,

_1950e

In previous work reported (1) lt was shown that sub critically heated C steel* base plate could be embrittled, as a result of weldlng In a zone adjacent to the weld0 This

evidence was obtained with changes in transitIon

temperature

(eccentric notch tensile and Charpy V-notch) and hardness0 The degree of embrittlement increased with IncreasIng (1)

solution temperature in the 1lO0-120C°F range, (2) cooling

rate, and

(3)

room temperature aging time and was

decreased

by accelerated aging0

In view

of the fact that quench-aging is belIeved

respon-sible for the zone of minimum ductility found in ship plate

*The designation !C refers to Steel C In the series of Ship Structure

Committee

Project"

Steels.

-2-weldments** _{in a region not heated above the lower critical} temperature at any time(i), _{it was considered advisable to}

investigate the maximum embrittlement possible in this grade of steel by quench-aging and how to minimize or eliminate it0

To this end, Charpy V-notch impact specimens were solution heat treated at the maximum subcritical temperature (1300°F), water quenched, and then aged for various periods of time in the 35°

to 11000F range. The embrittlement was ev.luated by

transi-tion temperature and hardness changes, supplemented by

microscopic examination.

MATERIAL

The semi-killed, ship plate steel (C Steel) used in the present work was the same 3/1-in. thick, as-rolled plate

(Plate II) which supplied specimens for the earlier work on subcritical heat treatment0 The properties reported for this

steel are as follows:

TABLE I

PROPERTIES 0F C STEEL PLATE Chemical Analysis Carbon 0,2+ Copper 0Q03 Manganese 0.18 Chromium 0.03 Phosphorous 0.012 Molybdenum 00005 Sulfur 0.026 Tin 0.003 Silicon _0.05 Nitrogen 0.009 Aluminum

0016

Vanadium 0,02 Nickel 0.02 Arsenic

0001

Mechanical Properties

Yield Point Tensile Strength Elongation

Psi Psi Per cent

j9,000 _{F7,'fC0 25.5 (8e' gauge)}

**At least for weldments made of A and C steels, the two

PROCEDtJBE

Ten impact blanks (0+2O" square) were cut from the as rolled plate so that the long axis of each specimen was

perpendicular to the rolling direction0 These blanks were

used as hardness check specimens after heat treating at 1300°F

for

₁₅

minutes in a neutral chloride salt bath*0 One

speeL-men was air cooled while the remaining nine were water quenched0 Each of the quenched specimens was then aged for various times

in an appropriate bath of water, oil, or tempering salt at one of the following temperatures: 350, room temperature (800),

125°,

2O0,

2500,

_350°,

_{600°, 900°, and 1100°F0 All specimens}

were air cooled froni the aging temperatures0

The as-quenched'1 hardness was taken in about five minutes after quenching0 In following the progress of aging, specimens

were removed from the

aging baths for

a time only long enough

for the hardness tests to be made0

With the hardness checks as a guide, series of Charpy

V-notch impact blanks were taken from the plate

in

the same

manner, quenched from 13000F, and aged for various times at

room temperature, 1250,

_35Q0,

and 600°F0 In addition, one series was air cooled from 13000F to give the unembrittled eondition0**

*The composition of the neutral chloride salt was asfollows:

BaCi2 _55% NaC1 _22%

ICC1 _22% "Dicy" _1%

**The Transition Temperature of this series was approximately

the same as that of the as.received material0 _{In addition,} hardness checks of broken specimens showed no change with

After aging, the specimens were immediately ground to

size and the notch cut perpendicular to the surface of the

plate, with each series being machined and tested within

seven hours after aging0 The impact testing procedure was

the same as given previousiy. As pointed out in the

earlier work, the possibility exists that accelerated aging

can occur in the testing bath0 In establishing the transi-.

tion curves, it was necessary to test as high as 350°F

(specimens were held 10 minutes in the bath to assure tempera-ture uniformity), and thus, accelerated aging in the test

bath may occur in those specimens previously aged at room

temperature and 125°F. Hardness checks made on specimens

from these series before and after breaking indicated that the

hardness was unaffected up to testing temperatures of

_175°F0

In view of the fact that the 15' f tJb transition temperature

is to be used as the criterion of embrittlement and in no case did this exceed 175°F, the results would appear to be unaffected

RESULTS Hardness Tests

The Rockwell B hardyìesses obtained after aging in the

35°

to 1100°F range are shown as a function of the aging time in

Fig. L

From an as-quenched hardness of R8?, the hardness of the

specimen aged at room temperature (80°F) remained unchanged

00 95 90 > 85 -J

'A

-J bi 80

o

o

loS -4

65 1250 F 2000F 250°F AS QUENCHED FROM 1300°F

ï1v+uIo'

AS- RECEIVED 7 6000F 900°F 1100°F 0.0_00 D_D . D.-

V.

ALL SPECIMENS AIR COOLED FROM AGING TEMPERATURE

/

\ 35°F _80°F 350°F 1 1000 10,000

LOG AGING TIME-1HOURS

FIG. i:

EFFECT OF VARIOUS AGING TIMES AND TEMPERATURES ON THE

HARDNESS OF IsOli STEEL AFTER WATER QUENCHING

FROM 1300°F.

x.AIR COOLED

FROM 1300°F o 0.1 IO. lOO

-o-level of RB96 after five days0 Aging at a lower temperature

(35°F)

resulted in a slower rate of increase and a longer

time (about 50 days) to reach a slightly higher peak level

of _RB9?o

With increasing aging temperatures above room temperature

to 250°F, the hardness increased at an ever increasing rate;

but the peak hardness reached and the time to reach this peak

decreased0 Once the peak had been attained, further aging time caused a decrease in hardness at a rate which increased with aging temperature0

At 350°F and higher the hardness did not rise above that of the as-quenched specimen but fell off at a rate which again

increased with temperature0

A summary of the maximum hardness reached and the time to reach this maximum for the various aging temperatures

employed are given in Table 110 TABLE II

EFFECT OF AGING TEMPERATURE ON THE PEAK HARDNESS REACHED AND TRE TINE TO ATTAIN

THIS PEAK

Aging Peak Hardness Time to Reach

Temperature, °F Reached, RH Peak Hardness

35

97

50 days

80

96 5 days

125

95

23 hours

200 91 2 hours

250

88

15

minutes

350

Not higher than as-quenched hardness of RB

87

t; 600

900

1100

Note: All specimens water quenched from 1300°F prior to

aging0 Hardness of specimen air cooled from

_1300°F

These results show that by aging the hardness can be raised

a maximum of about 10 points above that of the as-quenched

hard-ness (RB87)O In turn the as-quenched hardness was 17 points Rockwell B above that of the air cooled hardness (RE7O)o The maximum cumulative hardness increase due to solution and aging

then amounts to about 27 points REO

Iinact Tests

The impact transition temperatures by three criteria

obtained of the (1) as_received*, (2) air cooled, and

(3)

the various quench-aged conditions are summarized in Table IlL The individual transition curves for each condition are plotted

in

Figs0

2--7 with both the energy absorbed and the per cent

fibrous fracture plotted as the ordinate.

In the following sections of the report, the effects of the various subcritical heat treatments are evaluated with the 15 ft-lb transition temperature as the criterion of embrittle-ment and with the properties of the air cooled series reflecting

the unernbrittled state. The choice of either of the other two

criteria listed would reveal the same general effects0 In comparing the impact properties of the as-received

plate,

Fig0

2, with those of the air cooled series, Fig 3,

it Is evident that there is little difference in properties other than about a five ft-lb higher upper level for the air

cooled series0

The transition curves after aging for various times at *Previously reported (1)

CEARPY V-NOTCH TRANSITION TEMPERATURES AND HARDNESSES OF C STEEL AGED AT VARIOUS TEMPERATURES AND TIMES AFTER WATER QUENCHING

FROM _{13 00°F}

Transition Temperature,

50 Fibrous Rockwell B

Aging Tizne Mid.point* Fracture** 15 Ft_Lbs*** Hardness

As-Received *Tempera ture curve **Temperature temperature ***Temperature

TABLE III

NOTE: Only the aging times for impact specimens aged at

room temperature include the seven hours required for machining and testing0

at midpoint of absorbed energy-test temperature

at midpoint of per cent fibrous fracture-test

curve s

at 15 ft-lbs were absorbed.

118 138

Air Cooled from

100°F

85

85

75

70

118

11+5

Aged at oom Temperature

5 minutes

87

7 bourn

175

n S

88

3 days

i8o

212

158

91F 5 dayz 175 210 160 96 32 days

202

230

175

96 +5 days 175 96 Eed at

125°F

1 i/+ hours 150 200 127 91 7 1/2 hours

185

215

162

93 21 hours

188

218

168

9 3 days 182 208 ged at 50°F 168 9

15

minutes 133 160

115

87

i 3/1+ hours 125

160

100

83

19

hours

122

i1+8

gd

at00°

98

81

2 minutes 120

155

93 83 10 minutes

125

152

95

81

I-z

w O C.) w Q- ₂₀ 4O D I-

_o

4

1h. 80 u) D

o

loo -80

FIG. 2

CHARPY V-NOTCH

TRANSITION

CURVES

FOR AS-RECEIVED

"C"

STEEL.

o 20 40 60 80 (n D I00

80

L'.

-9-0 80 60 240 TEST TEMPERATURE --' 0 80 60 240 TEST TEMPERATURE ' 320 320 400

FIG. 3

CHARPY V-NOTCH

TRANSITION CURVES

FOR "C" STEEL

SUBCRITICALLY

HEATED

AT

300°F FOR

_{15 MINUTES AND}

_AIR

COOLED.

50 40 30 20 lo I-O

4

a. o

,

400 50 40 30 20 lo o s

.

.

>

,:

\

,Ip.._.

₀₀

SS'

./\

-10-room temperature, 1250, 350°, and 600°F, are presented in

Figs0

Lf.7,

respectively0 In all the quench-aged series the upper

level of the individual absorbed energy-test temperature curves

is lower than that for the air cooled series0 In general, a

greater difference in levels is associated with a greater degree of embrittlement, with the maximum difference being about 12

ft-lb0 The degree of ernbrittlement can be more readily seen

in the summary of the 15' ft-lb transition temperature plotted

as a function of the aging time, top of Fig. 8, for the various

aging temperatures employed0 For comparison purposes the com

parable hardness data are plotted in the bottom of this figure0

For room temperature aging (80°F), it can be seen that after seven hours aging the transition temperature (115°F) is

30°F higher than that of the air cooled series (85°F)0 With

increasing aging time, the transition temperature increases,

reaching a maximum level of 175°F at some time greater than five

days but less than 32 days0 This is an increase of 900F over

the unembrittled state0 Although no data were obtained for aging times less than seven hours (this was the minimum time

for machining and testing), it is not expected that the transition temperature would be appreciably lowered for the shorter aging

times because the hardness increased but one point during the first seven hours of aging0 The aging curve at the shorter

aging times has been interpolated to give an as-quenched9

20 40 60 80 00 20 40 o 60 80 _lOO o 20 40 60 80 _lOO

FIG. 4.(A-E):

CHARPY

FOR "Ce' STEEL 1300° F

FOR 15 AGED AT ROOM INDICATED. 50 4° 30 20 (O _o 50 ₄₀ 30 20 o 50 40 30

VNOTCHED TRANSITION CURVES SUBCRITICALLY HEATED AT _{MINUTES AND WATER QUENCHED.} TEMPERATURE FOR TIMES

o 20 40 60 80 _'oc o ,, 20 D o 40 u. 60 FIG. 4 CONTINUED 50 40 30 20 I0 o 5° 40 30 20 Io o

u

(A) - AGED 7 HOURS W1b o

-s.

'

/

3

-r

(B) AGED -DAYS

oo.o

'UtIO O

.

. o s I

O\

co-o-I---(C) AGED jo 5 O DAYS o.IuIIIIo. o coo I o

../

'4,

s.,

0l\

-!

(D) AGED 32 r I DAYS r i o

\\

/S

r'

4,. o -(E) AGED 45 DAYS Io _o 80 100 -80 o 80 60 240 400 360 400 320 -80 0 80 60 240 TEST TEMPERATURE -TEST TEMPERATURE"-F

a) 0 20 40 60 80 100 O 20 40 60 80 IOU o ₂₀ (n 40 o 60 80 loo o 20 40 60 80 _lOO o 50 40 30 20 Io o 50 40 30 20 40 lo o 50 40 FIG. 5: (A-D)

CHARPY V-NOTCHED TRANSITION CURVES

FOR

ICC

STEEL SUBCRITICALLY HEATED AT 1300°F

FOR

15 MINUTES AND WATER QUENCHED. AGED

AT 25°F FOR TIMES INDICATED. loO (A) AGED 15 MIN. (0) AGED o O O -f

--.4-1

o o I 3/4 HRS.± (G) AGED 9 HOS. . o

L

o 0O.0-° 20 TEST TEMPERATURE--F Io FIG. 6 (A-c)

CHARPY VNOTCHED TRANSITION CURVES

O

FOR "C" STEEL SUBCRITICALLY HEATED AT 1300°F

400

FOR

IS MINUTES AND WATER QUENCHED,

AGED AT 350°F FOR TIMES INDICATED.

t

50 40 30 20 - Io o 50 40 30 20 50 40 30 20 IO o (A) AGED I 1/4 1F'°'0hu1b HOURS

°L

._.-'i--i

/2 HOURS o o (B) AGED 7 00O -"aL . $

:l

I I S

t

--O___

I O O 0O 0' (G) AGED 21 . HOURS

o-0

0. o J

rv

S .4 's

-I

._

s....

o OOO 0 ID) AGED 3 DAYS O.r..O%OO -J. s-S-1..

S

_________

. .

Oo

-80 O 80 60 240 320 TEST TEMPERATURE -'-F 30 -80 O 80 160 240 320 400 50 40 30 20 20 40 Io 60 80 lOO o 20 40 60 80 60 80 lOO O 20

40 w o 60 Q. 80 w loo g-. o gr 20 u. 40 o gr o _{20 60} u. 80 00 50 40 30 20 Io o 50 40 30 20 Io o 200 190 80 170 w 160 gr I-. 150 -.1 w I. Q. g-. 140 u-w 'n'-30 C -FA R PV 125°F V- NOTCH

D

120----4 IO --660°F 7

90--AS RECEIVED OR AIR COOLED FROM

300°F 8oL* 350°F 00 80°F

U)I'

125°F--I AS QUENCED 8O°F 90 y ._- 350°F 85 80

L -AS RECEIVED

7Ox.-AIR COOLED FROM 1300°F

-65 60 IO IO,) 1000 10,000 LOG AGING TIME-s'HOURS FIG. 8

SUMMARY CURVES SHOWING EFFECT OF

VARIOUS AGING TEMPERATURES AND TIMES ON THE TRANSITION TEMPERATURE AND HARDNESS OF "C" STEEL. SPECIMENS WATER QUENCHED FROM 1300°F BEFORE AGING.

(A) AGED 2 I MIN

I,'

IO o-.00_.o IB) AGED I MIN. ._I_-Ir.l.-___ 0% -80 0 80 60 240 320 400 TEST TMPERATURE-'-°F FIG. T AB

CHARPY V-NOTCHED TRANSITION CURVES

FOR "C' STEEL SUBCRITICALLY

HEATED AT 1300°F

FOR

15 MINUTES AND WATER QUENCHED.

AGED

AT 600°F FOR TIMES

The transition temperature--aging time relationship for

125°F aging-- shows that the transition temperature starts to

increase at much shorter times, reaching a peak (168°F) after about 20 hours and remaining at this value at least up to

three days aging time0

For 350°F aging it appears that a slight peak may be

present at an aging time less than 15 minutes, but it probably

would not be much greater than the 115°F transition

tempera-ture obtained after 15 minutes aging because the hardness was

unchanged during this time intervaL Aging beyond 15 minutes

results in a decreasing ernbrittlement with time0

Aging at 600°F for two and ten minutes, respectively, effects a considerable improvement in the impact properties For both cases the transition temperature indicated an

embrit-tiement of but 10°F0

From the general trend of these aging curves, it is to be expected that at aging temperatures higher than 600°F the

impact properties would approach those of the unembrittled state very rapidly However, as will be pointed out in the Discussion, this conclusion only holds for specimens cooled

relatively slowly from the aging temperature; a fast cool from aging temperatures above about 6500 can introduce another

quench-aging cycle0 4i cro structure s

-1 5

quench-aging, a number ot specimens representing various quench

aged conditions were examined at 2000X.

The structure of the as-received condition is shown in Fïg0

9(a)0 No difference in structure was noted in the air cooled

condition; however, Immediately after quenching, a mottled

ferrite was evident which did not appear to change with time

at room temperature0 Fig0 9(b) shows the structure after one

month at room temperature No change in this mottled structure

was revealed after aging at 125°F, even after 23 days at this

temperature, Fig0 9(c).

After aging 15 minutes at 350°F, the mottled ferrite 'was still in evidence; after 21 hours at 350°F, Fig. 9(d), the mottled structure appeared to be more intense.

Upon aging at 600°F for two minutes, a precipitate, evenly distributed throughout the mottled ferrite grains, was

resolvable. An increase in aging time to ten minutes, Fig. 9(e), resulted in better definition of the precipitated particles,

which appeared to be platelets, and an increase in their size.

After aging ten minutes at 1100°F, Fig. 9(f), the precip Itate is no longer evenly distributed throughout the ferrite grains but appears to have coalesced along the grain boundaries

as spheroids.

DI SCUSSION

On the evidence of the hardness properties,

Fig0

1, aging curves were obtained which were characteristic of quench-aging

(a)

As-1eceived

(d)

Aged 21 Hours at 30°F

(b)

Aged 30 Day5 at Room Temp er ature

(c) Aged 23 Days at 12°F. 1 s

t m

(e) Aged 10 Minutes at 600°F (f) Aged 10 Minutes at 1100°F Fig. 9:

MICR0STRUCTU1ES 0F AS-PECEIVED AND VARIOUS QUENCH-AGED CONDITIONS.

J11 aging treatments preceded

by a water quench fran 1300°F.

2000X,

l7-systems, 10e0, the maximum hardness attained and the time to reach this maximum decreased with increasing aging temperature0

The similar changes observed with the impact transition

temperature, Fig0 8, are believed to be new experimental data0

The only information found on the impact transition temperature changes due to quench-aging in a similar grade of steel was

in the work by Low':6) _{and was restricted to the room} tempera-ture aging effects0 In this paper it was shown that after

quenching from 700°C (1290°F) the transition temperature

in-creased continually over the aging interval of three years0

The total increase amounted to a 110°F rise in Charpy keyhole

transition temperature (10 ft-lb value), with the greatest in-crease occurring in the first ten days aging0 This work was

done with a 1/2-inch hot rolled semi-killed plate containing

0l7% carbon0

In conjunction with the earlier work under this project,

it is now possible to show, Fig0 10, the effect of solution

temperature on the magnitude of the peak transition tempera-ture and hardness reached by room temperatempera-ture aging0 As is

evident, increasing the solution temperature in the 11000 to 1300°F range results in a greater initial (as_quenched!)

hardness and, apparently, a greater initial transition

tempera-ture0 _{Upon subsequent room temperature aging, the magnitude}

of the peak also increases with increasing solution temperature0

E 20 (n

z

110 loo 90 80 loo --V 95 90 70 65 X 60 o CHARPY V-NOTCH u AS RECEIVED RECEIVED

-18-85 L D D 80 -I3OO°F

o. 00

F o d00 1100°F 0,1 IO 00 1000 10,000

LOG AGING TIME -'- HOURS

FIG. io:

EFFECT OF ROOM TEMPERATURE AGING TIME

ON

THE TRANSITION TEMPERATURE AND HARDNESS

OF 'G" STEEL AFTER WATER QUENCHING FROM

TEMPERATURES

INDICATED.

200 90 80 40 30

z

o

i 9

which show that a greater change in properties can be expected with increasing solution temperature (greater degree of super

saturation)

To obtain a quantitative measure of the solid solution and the aging effects as a function of solution temperature

from room temperature to 1300°F, Fig0 il has been prepared0

Here for each property three heat treated conditions are under

considerations (1) air cooled, (2) as-quenched, and

(3)

quenched and aged one month at room ternperature The air cooled data

sets the base line for evaluating the two effects because in this condition it is assumed that the solid solution and aging effects are absent*0 The difference between the'as-quenched properties and the base line then yields the effect due to solid solution,

whereas the difference between the as-quenched and

'quench-aged' properties gives the effect due to aging0 The aging e-feet may be altered for aging times greater than one month, but it is believed that the change would be slight, if any, and therefore the effect shown may be considered a maximum0

This figure also shows that below a solution temperature of about 650°F the solid solution and aging effects are nil but that above this temperature both effects increase with increasing

solution temperature0 It is interesting to note that the hard-ness is affected to a greater extent than the transition tempera-ture by the solid solution effect, whereas the reverse is true

*Hardness checks showed no change with time at room

lOO 95 90 85 80 75 70 65 190 180 u-s 170 160 w 150

w

_{Q. 140} 130 20

l'o

100 90 80 AS REC,EIVED

'Io

AS RECEIVED o

-20-HARDNESS > AIR COOLED O _{AS QUENCHED (WATER)} QUENCHED AND AGED AT ROOM TEMPERATURE

FOR ONE MONTH.

CHARPY V-NOTCH AGING EFFECT°

\

SOLID SOLUTION

/

EFFECT

-I

AGING

_{- F-..}

EFFECT\ t.L r

u-

-w 0 200 400 600 800 1000 1200 400 SOLUTION TEMPERATURE

-FIG.

II

SOLUTION

AND ROOM TEMPERATURE AGING

EFFECTS ON

THE HARDNESS AND IMPACT

-21-with aging0 In any case the two effects are interrelated and

governed by the solution temperature which in turn controls

the amount of carbon available for the precipitation reaction0

With cooling rates intermediate to an air cool and water quench, it is to be expected that these two effects will be

minimized due to the lower degree of supersaturation, i0e., less

carbon is retained in solution on the quench0

From the metallographic changes occurring during aging,

it would appear that a two-stage precipitation reaction is

operative. The first stage (evident as a mottling of the fer-rite) was responsible for the greater degree of embrittlement,

while the second stage, in which a resolvable precipitate was detected, was associated with an improvement in properties

corresponding to a rapid overaged'condition0 Although no

attempt was ¡nade at identification, the sequence of micro-structural changes observed appear to be the same as reported recently

₍₇₎

In this paper the metallographic changes occurring

during the quench-aging of an ingot iron (0.026 carbon) were

followed by means of the optical and electron microscopes, and the identification of the two-stage precipitate made with

electron diffraction technique0 The first stage was identified

as the hexagonal E-iron carbide after

l5

hours at 200°C (390°F)0

The second stage was associated with a change in the crystal lattice to cementite, (orthorhombic Fe3C), observed after aging

-22-It can now be speculated that in the aging of C steel the formation of the transition lattice (&-iron carbide) is associ-ated with a high degree of embrittlement, while subsequent

formation of the equilibrium phase (cementite) brings about a large improvement in properties, corresponding to the rapid

overagedt condition0

Insofar as microstructural changes are concerned, the brittle zone previously found in the subcritically heated region in ship plate weidments has not been identified with a

precipitation reaction*0 However, due to the complexity of the temperature, time at temperature, and cooling rate condi-tions in multiple pass weldments, it is difficult to determine

the exact quench-aging cycle experienced. It has been shown previously (i) that the cooling rate in the subcritically heated region** is such that the ferrite is not supersaturated to the

maximum possible

as in ater quenched test specimens0

Conse-quently, less carbon is available for the subsequent precipita-tion reacprecipita-tion, and the resultant precipitate may not he detected***

In regard to the changes in hardness and transition tempera-ture, the results of this investigation give the maximum possible

*It should be recalled that the demonstrated beneficial ef-fects of increasing preheat (decreasing cooling rate) and postheating (overaging' certainly point to the

quench-aging mechanism0

**The cooling rate at the critical zone was found to be intermediate to water quenched and air cooled impact test specimens0

is not expected that the electron microscope would be

able to detect any precipitation0 In the reference cited

in the Discussion (7), the only advantage of the electron over the light microscope was in better resolution of the

-23-embrittlement of C steel by quench-aging, Fig. 11. Any

treat-ment designed to remove these effects in the base plate should be applicable to weidments made of this and similar grades of

Steele As was shown in Figs0 i and 8, overaging' treatments

served to minimize the ernbrittiement, with the degree of em brittlement decreasing with increasing aging temperature.

However, this finding must be qualified in view of the fact

that the results were based on specimens air cooled from the aging temperatures0 A consideration of Fig. ii shows that

if aging temperatures above 650°F were used, then the pos-sibility exists that another quench-aging cycle could be initiated if a fast cool were employed0

(Low6,

using a different approach, arrived at the same conclusion) There-fore, the postheating of weldments susceptible to quench-aging should be restricted to a maximum of about 650°F when this danger exists. As this investigation showed, aging in this neighborhood (600°F) effected a rapid and considerable improve-ment in the ductility by 'overaging.

CONCLUSIONS

In conjunction with previous work on the subcritical

heat treatment of a low carbon ship plate steel (C steel), the

following conclusions seem justified:

i _{The quench-aging mechanism was responsible for the loss in}

2 The severity of the embrittlement incrased with increasing solution temperature from

650°

to 1300 F; no embrittlement

was present when employing solution temperatures less than

650°F0

3 On the evidence of impact transition temperature and

hard-ness changes, characteristic aging curves were obtained,

Le0,

the peak transition temperature and hardness reached

and the time to attain this peak decreased with increasing aging temperature.

Metallographic examination showed that a two-stage

precipita-tion reacprecipita-tion was operative.

5 . low temperature postheat at about

650°F

would do much to

eliminate by overaging the zone cf minimum ductility

previously found in the subcritically heated region of weld-ments of this and similar grades of' steel0 Higher postheat temperatures run the danger of introducing another aging

cycle if a fast cool is employed

FUTURE WORK

This report concludes the experimental work done under this

contract. A final report will be prepared summarizing all the work done on (i) the distribution of relative ductility in ship

plate weidments and (2) the subcritical heat treatment of base

-25-BIBLIOGRAPHY

l.E0 B0 Evans, and L0 J. Klingler, "The Effect of Subcritical Heat Treatment on the Transition Temperature of a Low Carbon

Ship Plate Steel", Fourth Progress Report, Ship Structure Committee Serial No0 SSC-60, October 30, 1953.

2 G0 Sachs, L0 J0 Ebert, and A0 W. Dana, Jr0, "The Fundamental

Factors Influencing the Behavior of Welded Structures Under

Conditions of Multiaxial Stress and Variations of

Tempera-ture, Stress Concentration, and Rates of Strain", Navy Department, Bureau of Ships, Contract NObsJ+5+70, Serial

No0 SSC2+, May

10,

l9+9

3

L0 J.

Klingler, L0 J. Ebert and W M. Baldwin, Jr., Ibid,

2, Serial No0 SSC-3+, November 28, l9+9.

+. E0 B. Evans and L. J. Klingler, Ibid 2, Serial No. SSC-5+, October 1+, 1952.

Technical Progress Report of the Ship Structure Committee,

Welding Journal, Vol. 13 (July l98), pp. 377s-38+s0 J. R. Low, Jr., "The Effect of QuenchAging on the Notch

Sensitivity of Steel", Welding Research Supplement, Vol0 17 (May 1952), p. 253s-256so

Anna L0 Tsou, J. Nutting, and J. W Menter, "The

Çuench-Aging of Iron", Journal of the Iron and Steel Institute, Vol. 172, Part 2, October, 1952, pp. 163171.