Skip to main content

Full text of "DTIC AD0406103: THERMAL-MECHANICAL TREATMENTS APPLIED TO ULTRA HIGH STRENGTH BAINITES"

See other formats


UNCLASSIFIED 

. 1 , 4a(rio3 

DEFENSE DOCUMENTATION CENTER 

FOR 

SCIENTIFIC AND TECHNICAL INFORMATION 

CAMERON STATION. ALEXANDRIA. VIRGINIA 



UNCLASSIFIED 



JI I K 11 s I RI M 
( A\IHRIII(,I i" 
\1ASSA( [II'SI I IS 




NOTICE: Vfhen government or other drawings, speci¬ 
fications or other data are used for any purpose 
other than In connection with a definitely related 
government procurement operation, the U. S. 
Ckjvemment thei'ehy incurs no rosponslhlllty, nor any 
obligation whatsoever; and the fact that the Govern¬ 
ment may have formulated, furnished, or in any way 
supplied the said drawings, specifications, or other 
data is not to he regarded hy Implication or other¬ 
wise as in any manner licensing the bolder or any 
other person or corporation, or conveying any rights 
or permission to manufacture, use or sell any 
patented Invention that may In any way be related 
thereto. 




THERMAL- MECHANICAL 
TREATMENTS APPUED TO 
ULTRA HIGH STRENGTH 
BAINITES 


First Progress Report 
April 15, 1963 

by 

D. KaUsh 
S.A. Kulin 

Submitted to: 

Materials Branch 

Research and Engineering Division 
Bureau of Naval Weapons 
Department of the Navy 


Dj 

MATERIALS RESEARCH AND DEVELOPMENT IHmiLRBS, IIIC. 


21 ERIE STREET 
CAMBRIDGE 39 
MASSACHUSETTS 



ABSTRACT 


A program has been initiated to determine the effects 
of thermalomechanical treatments on the mechanical properties 
of high strength bainites. Particular concern will be paid to the 
strengthening mechanisms involved. The three alloys chosen for 
study are HI 1, 4340, and 4350. The pertinent transformation 
characteristics of these steels are being determined. 




TABLE OF CONTENTS 


Section Page 

I. INTRODUCTION 1 

Table I - Comparison of Mechanical Properties 
of Various Treatments . 2 

II. EXPERIMENTAL PROCEDURES 

A. Materials 3 

Table 2 « Chemical Compositions . 3 

B. Thermal and Thermal>Mechanical 

Treatments 4 

Fig. 1 « Schematic representation of thermal 

and thermal mechanical treatments ... 5 

Fig. 2 • Schematic representation thermal 

mechanical treatments 6 

C. Mechanical Testing . 8 

D. Structure Determination •.•••• 9 

in. EXPERIMENTAL RESULTS . 11 

A. Determination of M^ Temperatures .... 11 

B. Bainite Reaction 11 

C. Limits of Bay Region of TTT Diagram ... 12 

IV. FUTURE WORK 13 

REFERENCES .. 15 

i 















I. INTRODUCTION 


Significant improvements in the strengths of low alloy steels have been 
achieved by the application of mechanical deformation either before or after 
the formation of martensite. Ausforming; the deformation of metastable 
austenite prior to transformation to martensite has been shown to develop yield 
strengths over 300,000 psi in a number of commercial and experimental alloys. 
In addition, straining and the subsequent aging of as-quenched or tempered 
martensite ahjo develops exceptionally high strengths in low alloy steels. 

Both treatments, ausforming and strain aging have been applied to Hll die 
steel^^ and AISI 4340 alloy^^* The reported ductilities and resistance to 
brittle failure at these improved strength levels have, in general, been equal 
to or less than that developed by the conventional heat treatment. 

High strengths are a desirable but insufficient requirement for most 
applications. In addition to strength, the ductility and fracture toughness 
characteristics are also important. In this respect, thermal mechanical treat¬ 
ments involving martensitic structures as the final product do not offer much 
promise. If strength levels of 300,000 psi or greater are to be used it will be 
necessary either a) to improve fabrication and inspection techniques so as to 
eliminate all flaws that could cause premature failure or b) develop materials 
that possess sufficient ductility and toughness at these ultra-high-strength 
levels. 

In the search for methods to improve the strength, ductility, and fracture 
toughness of low ailloy steels the possible utilization of badmtic structures sub¬ 
jected to thermal-mechanical treatments has been virtually unesqplored. This 
laboratory has obtained a limited indication that significant improvements in the 



ductility and strength of HU can be achieved by combining thermal mechanical 
treatments with isothermal transformation to a bainitic structure. One 
process investigated involved deformation of austenite at temperatures in the 
bay region of the TTT diagram followed by a transformation to bainite, (referred 
to as ausbainworking). A second treatment examined involves strain aging an 
ausbainworked structure. The following table compares typical tensile 
properties of HI 1 subjected to various treatments: 

Table 1 

Comparison of Mechanical Properties of Various Treatments 


Treatment 

0. 2% Yield 
Strength, psi 

Ultimate tensile 
Strength, psi 

% elong. 
in 2 inches 

1, Conventional 
heat treatment 

240,000 

290,000 

5 

2. Ausforming 

215,000 

350,000 

5 

3. Strain aging 
martensite 

330,000 

330,000 

2 

4. Ausbainworking 

250,000 

345,000 

14 

5. Ausbainworking 
Strain aging 

and 

330,000 

346,000 

11 


It may be seen that the processes involving ausbainworking develop the 
optimum properties. 


This program was initiated to fully evaluate the effect of combining the 
bainite transformation with thermal»mechanical treatments. The influence of 
various processing parameters on tensile and fracture toughness characteristics 
will be determined. Emphasis will be placed upon investigating the strengthening 
mechanisms and the reasons for improved ductility in Hll. In addition, the 
investigation will be extended to the AISI 43XX class of alloys. 

. 2 - 



II. EXPERIMENTAL PROCEDURES 


A. Materials 

The low alloy steels to be studied in this investigation, listed in 
Table 2, were selected on the basis of the following considerations: (1) the 
connpositions are such that the steels may be deformed as metastable 
austenite and then transformed to bainite, (2) the tempering behaviors of 
the alloys in the conventionally heat treated condition are well defined; (3) the 
influence of thermal-mechanical treatments involving the martensite reaction 
has previously been studied and (4) the materials are commercially available. 
Two low alloy steels that meet these requirements are the 5 Cr-Mo-V alloy 
Hll and the Ni-Cr-Mo alloy 4340. The interesting results from the preliminary 
investigation of ausbainworking involving Hll suggests that further study of 
this alloy would be fruitful. A third steel, 4350, was selected for study in 
order to display the role of carbon content in the various thermal>mechanical 
treatments. The additional carbon will also provide a lower Ms temperature 
and thus allow a wider range of bainites to be formed than is possible with 4340. 
The alloys were obtained as CVM 2f' wide sheet in two thicknesses, 0.250^' 
and 0. 140*'. The thicker material will be used for experiments involving 
deformations of more than 20%. 

Table 2 

Chemical Compositions 
Element % 


Steel 

C 

Mn 

Si 

Cr 

Mo 

Ni 

V 

P 

S 

Hll 

0.39 

0. 25 

1.00 

5.25 

1.39 

m 

0.54 

0.011 

0. 007 

4340 

0.43 

0.68 

0.34 

0.83 

0. 24 

1.80 

- 

0.010 

0. 006 

4350 

0,52 

0.65 

0.22 

0.88 

0.26 

1.82 

• 

0.008 

0. 006 


3 



B, Thermal and Thermal-Mechanical Treatments 


The eight basic treatments being employed in this investigation are 
schematically shown in Figs. 1 and 2. 

1, Thermal Treatments 

Processes B and M (Fig. 1) refer to the formation of bainite or 

martensite, respectively, without any deformation being introduced into the 

heat treatment cycle. Bainite is formed by austenitizing for 30 min. in seilt 

at some temperature (T ), hot quenching into salt at the appropriate bainite 

reaction temperature (T ), holding to the end of transformation {t£) and oil 

quenching. Martensite is formed by oil quenching 43XX alloys or air cooling 

Hll from T . Both processes B and M are followed by a double-tempering 

T 

treatment (1 + 1 hour at T ) in salt baths. All material is air cooled from the 
tempering temperatures. 

It should be noted that all salt bath temperatures are controlled 
to within + 5°F, while air furnaces are held to + 10°F. 

2. Deformation of Austenite 

Processes AB and AM (Fig. 1) involve the deformation of meta¬ 
stable austenite before transformation to either bainite or martensite. Speci- 

A 

mens are hot quenched from T into a salt bath at the desired deformation 
temperature (T^). After 6 minutes in the salt the material is rapidly trans¬ 
ferred to an adjacent air furnace for two minutes at the same temperature (T^) 
in order to allow the salt to drain and evaporate off. Previous experience in 
this area has shown that rolling a dry sample from an air furnace is easier 
than rolling a specimen covered with salt. The specimen is then reduced by 
rolling in a 2-high 15 inch mill. The reduction in thickness is achieved in 4 
passes, with most of the deformation occurring during the first two passes and 


- 4 - 




SOmin-H ^AK30mia^ 



Fig. I-Schematic Representation of Thermal and Thermal Mechanical 
Treatments. 



- 6 - 


Fig. 2 - Schematic Representation Thermal Mechanical Treatments. 



the final two passes used prinnarily to straighten the blank. Because of a drop 
in tennperature during the rolling operation, specimens are returned to the 
salt pot (at T^) for three minutes after each pass and then trainsferred to the 
air furnace (at T^) for two minutes in order to reheat them to the initial deforma¬ 
tion temperature. The surface temperature of specimens leaving the rolls, as 
determined by a contact-pyrometer is approximately 100°F below the tempera¬ 
ture of the furnace. The total time at the deformation temperature is twenty- 
three minutes. The large majority of experiments involving austenite deforma¬ 
tion will have a constant percent reduction in thickness of 50%. The 0.250^ stock 
will be used and thus reduced to 0. 125'*. Following the last deformation pass, 

g 

the specimens will be either hot quenched to T in order to form bainite or oil 

quenched to form martensite. Processes AB and AM will be completed by double* 

T 

tempering (1 + 1 hour at T ). 

3, Strain-Aging Martensite and Bainite 

Processes BS and MS refer to straining bainite and martensite, 

respectively. After the formaition of bainite or martensite the structures are 

T1 

single-tempered (1 hour) at some temperature (T ). At this point deformation 
may be introduced at room temperatures either by rolling the original blank or by 
straining a machined tensile specimen. Although a wider range of reductions can 
be obtained by rolling, the tension method provides a more homogeneous deforma* 
tion. These two methods will be compared further in order to select the most 

desirable method of straining. Following the deformation the structures are 

T2 T1 T 

retempered (1 hour at T , where T is not necessarily the same at T 

4. Combination Thermal«*Mechaiiical Treatments 

Processes ABS and AMS (Fig. 2) combine deformation of the parent 
austenite phase with straining of the subsequently formed transformation product 



- 7 - 




(either bainite or martensite). Before these treatments are investigated, the 
influences of a number of the processing variables on the resulting mechauiical 
properties will be determined for the less complex treatments. As indicated 
in the data from the preliminary studies of Hll, it may be possible to develop 
desirable properties by combining simple thermal-mechanical treatments. 

5. Refrigeration Treatments 

For various treatments it will be desirable to introduce subcooling 
in order to determine the role of retained austenite on the structure and proper¬ 
ties, Refrigeration wotild generedly be in liquid nitrogen following: (1) the quench 
from either T^, or T®, depending upon the particular treatment and (2) 

the first tempering treatment. Particular treatments may adso require 

g 

variations in the subcooling temperature (T ) in the range between room tempera¬ 
ture amd liquid nitrogen, 

C, Mechanical Testing 

1, Hardness and Tensile 

The hardness and unnotched tensile properties of the various 
bainitic and tempered martensitic structures will be determined. The para¬ 
meters that will be obtained from the tensile test are the yield strength, ultimate 
tensile strength, elongation and reduction in area. 

2, Fracture Toughness 

The precracked subsize Charpy test^^^ has been chosen for evaluatinf 

fracture toughness. This laboratory has considerable experience in the use of 

(2 3 7) 

this test; particularly in the field of high-strength steels ’ ' . It has been 

found that this test is highly sensitive, reproducible, economical, and relatively 
easy to perform and evaluate. The parameter measured will be the energy per 

unit area, W/A, required to propagate a crack. 


- 8 - 



The dimensions of the subsize V-notch Charpy specimen will 
be 2.165 inches in length, 0.394 inches in width and 0.080 inches in thick¬ 
ness. Precracks, about 0.015 inches in length and through the thicknesses, 
will be produced in a fatigue apparatus developed for this purpose at ManLabs. 

In order to investigate the effect of strain rate on fracture toughness, both 
impact and slow bend tests will be performed. Impact or slow bend energy 
absorption will be measured when necessary as a function of test temperature 
for various treatments. 

The currently most widely selected criterion for fracture tough¬ 
ness in sheet is the critical crack extension force, G , obtained from the 
Irwin-Kies center-notched tensile test. Experimently, the test is quite 

difficult to perform, particularly at various temperatures or at impact rates 

« 

of strain. The specimens are expensive in preparation time and materiad. 
Nevertheless, the fact that the results are quantitative auid many materials 
have been evaluated with this test makes it desirable to obtain G. values for a 
few conditions of particular interest in this program. 

D. Structure Determination 
1. Metallogr^hy 

Light and electron microscopy will be utilized in order to correlate 
the observed mechanical properties with such micro structural features as prior 
austenite grain size, carbide and bainite morphology, and volume fraction of 
the various constituents present. 

The times required for the initiation (t^) and completion (t^) of the 
bainite reaction will be established by metallographic examination of isothermally 
reacted samples. The temperature of each alloy for various austenitizing 
conditions will be determined by the Greninger-Troiano technique. An accurate 


- 9 - 



measurement of is essential since bainite reaction temperatures very close 
to are to be employed. 

2, X-Ray 

Changes in retained austenite content will be determined by the 
integrated intensity x-ray technique using monochromatic x-radiation and a 
scintillation detector. Measurements will be made on the effect of refrigeration 
and straining on the austenite to martensite transformation. 


10 - 



m. EXPERIMENTAL RESULTS 


A, Determination of M , Temperatures 

The martensite start temperatures of the three alloys have been 
experimentally determined by the Greninger-Troiano metallographic technique. 
The following temperatures were obtained for the indicated austenitizing 
temperatures (T^: 

Hll - M ■ 555°F for T^ * 1850°F 

8 

4340 - a 590°F for T'^ a 1550°F 

4350 - a 515°F for T^ a 1550°F 

The influence of austenite deformation on the initiation and progress 
of the martensite reaction is being determined. 

B, Bainite Reaction 

Hardness specimens have been heat treated in order to determine the 
progress of the bainite reaction in 4350 and Hll. Coupons of 4350 have been 
austenitized at 1550°F, hot quenched to temperatures in the range of 550»900°F, 
in 50^F intervals, held for times from 1 min. to IZ hours and oil quenched. 
Blanks of Hll were austenitized at i850°F, hot quenched to temperatures in the 
range of 550 - 700°F, in 50^F intervals, held for times from 1 minute to 64 
hours and oil quenched. Published transformation diagrams of Hli indicate 
that: (1) the entire temperature range for bainite reactions is lower than for 
43XX type alloys, and (2) no significant degree of isothermal transformation 
occurs in reasonable times at temperatures above 650°F, 

Electron microscopy of selected samples will be employed to determine 
the type of bainite formed at various reaction temperatures. 


11 - 





C. Limit* of Bay Region of TTT Diagram 

A particiilar advantage of Hll» with respect to metastable austenite 

deformation processes, is that the bay region of the TTT Diagram is quite 

o 4 

extensive, (700 to 1100 F for times beyond 10 minutes). From the literature 
it appears that the 43XX alloys are not as suitable for such types of thermal- 
mechanical treatments. Without deformation, the bay region of these steels is 
limited to temperatures in the range of 950 to lOSO^F and is limited to times of 
the order of 10^ minutes. In view of these facts the limits of the bay region of 
4350 are being accurately determined by hardness and metallography. The 
effect of deformation on the limits of the bay region is also of particular concern 
since multipass rolling may be desirable. 


12 




IV. FUTURE WORK 

The effort in the program will be divided between the following two main 
objective!: (1) determining the effect of processing variables on resulting 
mechanical properties for the various thermal and thermal-mechanical treat* 
ments and (2) relating structure to properties in an attempt to understand both 
strengthening mechanisms and the parameters that affect ductility and fracture 
toughness. Objective (1) will be concentrated upon during the first six months. 
The treatment variables of interest are given in the following outline: 

A. Determine Transformation Characteristics of the Alloys. 

1. Mg temperature: with and without austenite deformation. 

2. Progress of bainite reaction; with and without austenite deformation. 

3. Effect of deformation on limits of bay region of TTT diagram. 

B. Processing Variables • 

1. Processes B and M (Fig. 1). 

a. austenitizing temperature (T^ 

T 

b. tempering temperature (T ) 

g 

c. bainite reaction temperature (T ) 

d. degree of transformation 

e. subcooling temperature (T ) 

2. Processes AB and AM (Fig. 1} 


a. austenitizing temperature 


(T^) 


4b. tempering temperature (T ) 

c. bainite reaction temperature (T ) 

d. degree of transformation 

g 

e. subcooling temperature (T ) 

f. percent deformation of austenite 


13 - 





3. Proceiies BS and MS (Fig. 2) • 

T1 

a. pretempering temperature (T ) 

b. retempering temperature 

c. percent deformation of martensite or bainite 

4. Combination Treatments ABS and AMS (Fig. 2). 


- 14 



References 


1. V. F. Zackay, W. M. Justusson and D. J. Schnuitz, "Deformation of 
Metastable Austenite", Metal Progress, 80 No. 3, (Sept. 1961), p. 68. 

2. P. Stark and D. Kalish, Unpublished Work, ManLabs, Inc. 

3. P. J. Fopiano, S. Das Gupta and D. Kalish, "Effect of Mechanical and 
Thermal Processing on High Strength Steels", ManLabs, Inc., Watertown 
Arsenal Laboratories Final Technical Report No. 320.4/4-3 (June 1962). 

4. E. B. Kula and J. M. Ohosi, "Effect of Defornnation Prior to Transforma 
tion on the Mechanical Properties of 4340", Trans. ASM ^ (I960), p. 321 

5. £. T. Stephenson and M. Cohen, "The Effect of Prestraining and Re- 
Tempering on AISI Type 4340", Trans. ASM^ (1961) p. 72. 

6. G. M. Orner and C. E. Hartbower, "Sheet Fracture Toughness Evaluated 
by Charpy Impact and Slow Bend", Welding Journal Research Supplement, 
(Sept. 1961). 

7. S. V. Radcliffe, M. Schatz, G. Orner and G. Bruggeman: "The Flow 
Tempering of High Strength Steel", ManLabs, Inc., Watertown Arsenal 
Laboratories Final Technical Report No. 320.4/3-1 (April 1962). 


- 15 -