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Proceedings of the ASME 2010 Pressure Vessels & Piping
Division Conference PVP2010
July 18-22, 2010, Bellevue, Washington, USA
PVP201 0-2527 4
POTENTIAL DETRIMENTAL CONSEQUENCES OF EXCESSIVE PWHT ON PRESSURE
VESSEL STEEL PROPERTIES
Cedric Chauvy ArcelorMitlal lndusteel Chil.teauneuf, France
Lionel Coudreuse ArcelorMitlal lndusteel Chateauneuf, France
Patrick Toussaint ArcelorMitlal lndusteel
Charleroi, Belgium
ABSTRACT During fabrication of Pressure Vessels, steels
undergo
several heat treatments that aim to confer the required
properties on the entire equipment, including welds and base metal.
Indeed, the Quality heat treatment of the base material, which
leads to achieve the target properties, is most of the time
followed by Post \Veld Heat Treatment (PWHT). The aim of such
treatments is to insure a good behaviour of the welded zones in
terms of residual stresses and obviously properties such as
toughness. Generally, many simulated PWHT (up to 4 or more) are
required for the testing of the base material, which can affect its
properties and even lead to non acceptable results. In some cases
for fabrication purposes an intermediate Stress relieving treatment
can be required
Special attention is paid on CHMn steels (e.g. SA/A516 from ASME
BPV Code) with the effect of thickness and Ceq (11\V Carbon
equivalent fonnula: see page 3) requirements on the final
compromise between properties and heat treatments. In particular,
toughness and UTS are the critical parameters that will limit the
acceptance of too high P\VHT. Although micro~ alloying is a mean to
increase the resistance to P\VHT, this leads to difficulties in
softening the heat affected zones. This solution is therefore not
the best one considering the whole equipment optimisation. Finally,
the manufacturing process can play a major role when specifications
are stringent. Quenching and tempering can indeed provide better
flexibility in terms of PWHT and improved toughness for given Ceq
and thickness.
The case of Cr~Mo(-V) steels, which are widely used in the
energy industry, is also addressed. Indeed, P\VHT requirements for
increasing the toughness in the weld metal can lead to decrease the
base metal properties below the specification limits. For example,
the case of SA/A387grll is very typical of metallurgical changes
that can occur during these high P\VHT leading to a degradation of
toughness in the base metal. Another focus is made on the Vanadium
Cr-Mo
grade SA/A542D that must withstand very high PWHT (705C and even
7l0C) because of welds toughness issues. Optimisation has therefore
to be done to increase the resistance to softening and to guarantee
acceptable microstructure, especially in the case of thick wall
vessels.
Some ways for improvement are proposed on the basis of the
equivalent LMP tempering parameter concept. The basic philosophy is
to fulfil the need for discussion between companies involved in
pressure vessels fabrication so that the best compromise can be
found to ensure the best and safest behaviour of the equipment as a
whole.
INTRODUCTION Plates and other components of a pressure vessel
usually
undergo various heat treatments in order to make them meet the
requirements, which may be either customer specifications or
intemational standards. The base metal before welding can thus be
normalized, normalized and tempered or quenched and tempered.
During vessel fabrication, the manufacturer performs several
thermal cycles: preheating, post heating, DHT (De-Hydrogenation
Treatment), ISR (Intermediate Stress Relieving), PWHT (Post \Veld
Heat Treatment). Due to the temperature range, only JSR and PWHT
can be considered as real heat treatments.
Usually, it is considered that tempering confers the mechanical
properties on the base metal and the subsequent heat treatments
confer the properties on the weld area. In particular in case of
repairs, it is not unusual to perform several cycles of PWHT: up to
3 or 4. Actually, all these thermal cycles will affect more or less
the properties of the base metal. This becomes stili more critical
for thick wall vessels for which long PWHT duration can be
necessary.
In order to minimize the effect of P\VHT on the base metal
mechanical properties, the steelmakers are frequently
Copyright 2010 by ASME
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asked to perform the tempering (if required) at higher
temperature than the PWIIT, which focuses on the weld metal.
Moreover, the mechanical tests have to be carried out after
simulated P\VHT (Temperature above 454C according to ASME or other
code) in order to prove that the base metal can withstand these
thennal cycles.
In many cases, the PWI-IT has a real influence on the base metal
properties and the stcehnakers have to take it into account while
defining the grade and the manufacturing route. For instance, in
case of thick wall reactors (> lOOmm) which require PWHT at high
temperature during long periods, the steel supplier must sometimes
decline the tempering temperatures required by the customer
specification. Indeed, it is sometimes not possible to guarantee
the required properties after PWHT if using a too high tempering
treatment for a given grade.
Genera11y, the most convenient way to quantify the effect of
several thermal cycles on the properties is to use time-temperature
equivalent laws or tempering parameters. For pressure vessel
steels, the parameter commonly used is the Larson Miller Parameter
LMP (also called Hollomon-Jaffee Parameter).
The goal of this paper, after a brief rehearsal on the tempering
parameter, is to show the limits in term of tempering and PWHT of
different steel grades. Especially in case of heavy gauges, the
final properties arc not only given by the tempering step but by
the whole thermal history that has to be applied to the
material.
EQUIVALENT TEMPERING PARAMETER: LARSON MILLER PARAMETER
The influence of time and temperature on mechanical properties
is well known. Figure I gives an illustration of typical curves
that can be drawn from experience (for a 2.25Cr-lMo steel grade in
that case). Nevertheless, it is difficult to compare the effect of
different heat treatments, made at different temperatures and
different durations, and it is also impossible to determine the
cumulative effect of two or more heat treatments at different
temperatures.
600
700
;urs {TeniP-63-S'C) UTS (Temp 665-cf ~ UTS (r.imP 690'C)' ~YS(Te
-~35'C) oYS{Temp6SS"C) t.YS(Temp690"C) j
soo+---~ " 500 {} ... -
- "' ... ~- . ' 400
300 10 100
Tlme(h) Figure I: Influence of temperature and time on
tensile
properties (grade A387gr22 cl2). Therefore, the usc of an
equivalence parameter, which
involves both time and temperature, is of great interest. The
most used tempering parameter is called the Larson-Miller Parameter
LMP (or the Hollomon-Jaffee parameter).
The Larson-Miller Parameter formula is given hereafter for a
single heat treatment:
LMP=B(20+1og(t)) (I)
with: 8 temperature (K) and t time (hours)
It is also possible to extend this relationship to a multiple
treatment. In that case, the method is to choose one temperature 8
(usually the one used for the first treatment) and to calculate for
each treatment an equivalent duration teq at temperature 8, by
equalizing the LMP between the set of conditions ( 8, teq) and (8,,
t;).
These durations have then to be used in formula (1) and this
leads to the following general expression:
LMP,1 = 8 [ 20+ log( ~1,11 J] (3) Bis the chosen reference
temperature (K) t .. qi is the equivalent duration at 8 for a
treatment i made at B, during t1 (hours)
Figure 2 below is an example of the relevance of such a
parameter. It shows a lot of data obtained with a single treatment
(blue dots) but also some results from multiple treatments (red and
green dots within red circles).
[.-uTsonH)'o;.\e YSonec,ode --- - 'uTi(sOO:.C.shw+69o:lOC:i4hJ1
~'@_(~'C~ t 690'C-2-il\) UTS(710'C-6h + 690'C-33h) o VS
(710'C-61>+ 690'C-33h) !
2
800 ~---------
600 +~~~--~--~-~-a. .
:a: 5oo
400
300 +-----------19000 19500 20500 21000 21500
LMP
Figure 2: Mechanical properties as a function of LMP (grade
A387gr22 cl2)
Once this parameter is defined, it is possible to study the
Copyright 20 I 0 by ASME
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effect of cumulative heat treatments, which can be tempering,
ISR and P\VHT, on the different properties required for the
application (mainly tensile characteristics and Charpy V -Notch
toughness).
C-MN STEELS FOR PETROCHEMICAL APPLICATIONS The case ofC-Mn
steels such as A516 is very interesting
in that sense that many parameters must be taken into account
when considering the effects of P\VHT on materials properties.
First, the composition is often controlled through the use of a
maximum Carbon Equivalent Ceq. This parameter aims to ensure a good
weldability and is commonly defined by using the ll\V formula given
hereafter:
(4)
There are therefore a lot of clements that are not included in
this concept but allowed in the composition according to A516 or
A20. \Ve will see later how some other clements can be used and
their limitations.
Then the target is to obtain an acceptable microstructure,
matching the properties requirements, with a given chemical
composition. For a given manufacturing route, the cooling rate for
higher thicknesses will be lower than for the thinnest ones. In
some cases, particularly for heavy gauges, ensuring that the
properties are met implies to adapt either the Ceq value or the
cooling process.
Finally, depending on fabrication conditions, more or less
strong PWHT can be required leading to potential problems regarding
minimum values of mechanical properties.
In few words, for a given thickness, A516 must simultaneously
satisfy requirements of:
Ceq PWHT UTS and YS Charpy V -Notch impact values
Most of the time, A516 is delivered in normalized condition. For
thickness above lOOmm, this can lead to undermatch the target
values for mechanical properties in case of too high P\VHT. Then
the only alternative, for a given manufacturing route, is to
increase the level of Ceq as shown in figure 3.
This leads to PWHT lintitation of typically 610"C+/-IO"C for
15h. In other words, guaranteeing tensile properties in conformity
with the A516gr70 standard implies to limit the tempering parameter
value at approximately 18500 and 18900, for a Ceq of 0.43 and 0.45
respectively.
Influence of tempering parameter on UTS values
550 .-------540 t-------530 520
'i 510 e 5oo
5 ::~ 470 460+----450
17500 18000 18500 LMP
19000 19500
Figure 3: Influence of tempering parameter on UTS values for
A5!6gr70.
It is also important to note that in the case of C-Mn steels,
too strong PWHT leads to deterioration of both the toughness and
the tensile properties. Indeed, toughness is decreasing for
normalized C-Mn steels when increasing the global tempering
parameter and/or increasing thickness as shown in figure 4.
Depending on targeted values and temperature, this can sometimes
give the limitation in terms of acceptable PWHT.
200.0 ,------- -
175.0
150.0
v 125.0 ~ ~ 100.0
75.0
50.0 t-----=-
-
Steel without tlb /V (CarEisoe 70)
~----~' "~ ' I'\~ 24BHv
'-"; ~. r-"6 8
180
""\'J ,99 kJAnm -er 2, 78 kJtnm ---_:- 1,98 kJ/mrn- -- Maximum
a/leAved
Figure 5: Influence ofPWHT on hardness values for A516gr70
without micro-alloying elements.
The curves of figure 5 show that, for grade A516gr70, one can
guarantee hardness in HAZ lower than 248Hv with a stress-relieving
heat treatment at low temperatures (600C). It is to note that the
recommendation of NACE, which indicates that the PWHT should be
performed above 6200C, is not justified for C-Mn steels. In other
words, the minimum temperature to be applied is the one allowing
respecting the 248Hv maximum Heat Affected Zone Hardness while
respecting the minimum stress relieving temperature as per the
construction code, which is 595C (1100F) according to ASME VIII div
I and div2. H2S resistance, which is the concern of the NACE
recommendation, is already obtained using 600C.
Steel with Nb /V (type BS1501.225}
~ 1340 ~ ~ ' 1: ~
"" ..c ~300
"-. E~ \%:' -----,~ ~~ cr ~ ~ -g260 E./j 248Hv ~ -g220
:::>
180 ~ ~ ~
ou
" 8 8
-
Moreover the Q&T plates show significantly better toughness
and even a better evolution of this toughness with increasing LMP.
Figure 8 here below illustrates this behaviour.
160 -- -- --------
I I, .~-~~ CVN Q&T
I ~ t I . g_.-. - -":-._o_ lo t ~.o o ---~~-
0 -- I oo_o -
-J ----140
120
100
~ d so
60
40
20
15000 16000 17000 18000 19000 LarsonM~ler Parameter
Figure 8: Evolution of CVN toughness at 0C for A516gr70 (Ceq
0.43%) as a function of heat treatment and LMP.
As a conclusion, more and more stringent requirements show up;
leading to the need of finding always more accurate compromise
between properties and fabrication. There arc several parameters to
be used but each of them has its own limitation regarding the
application. For the thickest range of products (>l50mm), modem
stringent specifications can lead to usc quenched and tempered
steels that show higher performance levels, especially in terms of
toughness.
Whatever the application, discussions are always needed between
the actors involved in the fabrication chain, in order to best
define the good compromise for these high quality C-Mn steels.
1%CR - Y,MO STEEL FOR HIGH TEMPERATURE REACTORS
This CrMo low alloy grade (A387grll) is widely used in
manufacturing high temperature reactors for refining I
petrochemical applications. Indeed, it exhibits good mechanical
properties for operating in the typical range of 350-500C.
Chemical compositiOn does not confer enough hardenability on the
grade. For thin plates, normalizing can be sufficient to achieve
suitable microstructure whereas quenching and tempering is often
mandatory when increasing the thickness. This means that for thick
products (typically thickness >80mm) a significant amount of
Pearlite and Ferrite can be created even for quenched and tempered
material, and can be detrimental for toughness.
Moreover, after long exposure at temperature above 650C, there
can be a modification of carbides structure that may decrease both
strength and toughness. This is linked to globalization and
coarsening of existing carbides, which is called Upper Nose
embrittlement and is known since the 50s [1-
3]. Therefore one has to be very cautious because the
combination of tempering with Post Weld Heat Treatment may lead to
this situation. In general, the H4Cr- Y2Mo grade is to be used very
carefully when targeted thickness is above 75mm. In particular, it
can be helpful to reduce the tempering in order to get a larger
margin for PWHT that aims to confer toughness on welds.
Figures 9 and 10 illustrate respectively the evolutions of the
tensile and toughness properties as a function of the tempering
parameter LMP for A387grll. These results have been achieved from
SO and 143 mm thick plates with comparable chemical analysis. The
tempering parameters have been calculated for different
combinations of temperatures and holding time. (temperature from
660 to 740C; holding time between 30 min and 38h).
[ .. urS-sOmm thick "YS SOmm thick ur$ 143mm thlc_k __
,_ys_11~~mthh::kJ 700 r-~~~cc~c~
. .. -
!!'.""' " 450 -1----lt~-- },:
400 ::f===~~~~'"~,~~~~~'!~~~.'~c~'"~=-~3~~,~~~~~----~:_~--~ ""
+------------
19000 19500 ,0000 """' ""' """ LMP
Figure 9: Evolution of tensile properties of A387gde II cl2 as a
function of the tempering parameter.
Figure 9 shows that the tensile properties required for the
grade can be obtained for tempering parameters up to 21000. There
is no significant difference between both thicknesses. For that LMP
range, tensile properties are not the limiting element for that
grade.
f" 'Kv (:19-.C)_ SOmm thlck o Kv (-?9~1_1_43mm lhlcij 400
-------------------
"' "' -1-----'--~250 e;roo g w "'
100 ' --~'~------~-0
' '
50 +----------- -\ ~ ~ ~--- ~ ~ ~~~--~ ~~--~~~~~--~-~--~
'"""
19500 201100 ""' """ ""' LMP
Figure 10: Evolution of toughness (Kv -29C) of A387gdellcl2 as a
function of the tempering parameter.
5 Copyright 2010 by ASME
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On the other hand, figure 10 shows a different behavior for
toughness properties at -29C, which get worse beyond a given
tempering parameter value. Moreover, the thickness plays a really
detrimental role on the results and have thus to be taken into
account. It appears that impact properties and thickness constitute
the limiting clements to the tempering parameter for E4Cr- 'llMo
steel. As a matter of fact, the more important the thickness is,
the less there wi11 be possibilities to perform tempering
treatments and PWHT at high temperatures.
Then, for the thickest products, steehnakers are often conducted
to decline customer specification requirements. \Vhen the tempering
parameter calculated following the requirements does not allow
guaranteeing impact properties, there is never a single answer that
can be made. Therefore, there is a need of discussion between the
different concerned parties in order to define the best adapted
solution in tenus of compromise between PWHT, LMP, tempering and
Charpy V-notch requirements. If ever the requirements are
mismatching the material feasibility it is then necessary to choose
another material. For thick vessel, when low temperature toughness
properties are actually required (-l8C; -29C) with the use of
strong PWHT, the use of A387gde22 c12 is a safer alternative.
2'14CR - 1MO STEEL FOR HIGH TEMPERATURE REACTORS
2%Cr- I Mo type steel is also used for petrochemical reactors in
the same temperature range as for A387grll.
To the contrary of JlACr- lhMo steel, the hardenability is large
enough to obtain a suitable microstructure (that is to say without
ferrite) for thickness up to more than 200mm as far as a quench and
temper process is used. This kind of grade is therefore more
comfortable with regard to PWHT/toughness combination and toughness
is indeed not a usual issue. Strength will thus be the limiting
property when increasing PWHT.
Figure 11 illustrates the influence of the tempering parameter
on tensile strength of 21;4Cr-1Mo steel (A387gr22), for plates up
to 250 mm thick. On this figure, the lower thresholds of UTS
requirements are specified according to different standards of
214Cr-1Mo steels (ASTM and EN). Chemical compositions for these
different standards are the same; that is only by modifying the
heat treatment that it is possible to have an effect on the
mechanical properties.
2-25Cf11.lo sloe! so!t..-.!ngwrw
larson Miller Parameler
Figure 11: Influence of tempering parameter on tensile strength
of 21;4Cr-1 Mo steel.
ln order to guarantee the properties, a maximum value of the
tempering parameter can be determined depending on the
corresponding standard. The table below gives the maximum tempering
parameters for satisfying the tensile strength requirements from
the different standards; the maximum tempering temperatures and
PWHT conditions (for 200mm thick plates) arc also reported. It
appears that the choice of the standard will strongly affect the
amount of PWHT allowable on the grade. The lower the minimum UTS,
the higher could be the Larson Miller Parameter and therefore the
more there will be possibilities for increasing the total amount of
PWHT.
Standards LMP Tempering PWiiT maxi
-
A387 gde22 c/2 20850 710"C 690"C -33h
ENI00282 20550 660"C 680"C- 33h 12CrMo910
A542B c/4 20000 650"C 655"C -33h
Table 1: Examples of heat treatment hmllations as a function of
the 2%Cr-l Mo standard used.
The tempering parameters given in the above table have been
calculated for tempering times leading to a homogeneous temperature
through the whole thickness of the product. In fact, there is often
no tempering times requirements. Therefore, it could be possible to
reduce the tempering parameter by reducing the holding time.
However from a metallurgical point of view, this can produce
heterogeneities of properties though the thickness that can have
consequences during the fabrication of the vessel.
For example, tempering at 720C during I h leads to a tempering
parameter of 20,000. The same tempering parameter can be obtained
with a tempering at 690C during 5h30. Whereas, for a 50 mm thick
plate, there is no problem to perform a tempering treatment at 720C
for 1 h, it is not the case for a 250 nun thick plate, for which it
is better to apply a
6 Copyright 2010 by ASME
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tempering of 690C for 5h30 in order to homogenize the thermal
effect through the thickness. Therefore, the LMP approach must not
be applied without taking into account the specificity of thick
plates. There must be production rules that ensure thermal
homogeneity of the products and thus give the boundaries to respect
in terms of heating rate and holding time.
2Y,CR- 1MO- V.V STEEL FOR HIGH TEMPERATURE REACTORS
21,4Cr-1Mo-JAV steels (SA 542DCI4a or equivalent) arc more and
more used in refining installations for high hydrogen pressure and
high temperature ranges. Indeed, using a 21.4Cr-1Mo-%V grade
instead of 2%Cr-1Mo allows for a significant reduction of
thickness, especially since the new 2007 ASME VIII division 2
issue, which increased the a11owable stresses. The interest has
thus grown with the size of the newly designed reactors. Another
important advantage for the final users is weld overlay hydrogen
induced disbanding resistance. Indeed, this Vanadium enhanced grade
does not exhibit some hydrogen induced disbanding problems at the
plate-overlay interface.
However, once again, in order to obtain the required properties
in the welds, especially the Charpy-V impact toughness, the vessel
fabricator needs to perform PWHT at high temperatures (typica11y
705C and probably 71 0C in the near future). This leads that grade
to be able to withstand such high temperature PWHT during sometimes
more than 30h. Considering the trend to increase these PWHT
requirements, some optimization studies were necessary in order to
improve the resistance to softening. There are two ways to reach
that goal. The first one is to play on the microstructure itself
whereas the second one is to add some chemical elements that will
create stable precipitates resisting to softening.
Figure 12 illustrates the effect of different alloying elements
on the softening curve and shows how it is possible to guarantee
tensile strength properties of A542D cl4a for very thick products
(>l50mm), and for high tempering parameters by using
micro-alloying. On one hand, Boron promotes hardenability and
therefore improves the microstructure of the steel, especially for
thick products. A full bainitic microstructure can be achieved at
mid thickness up to 300nun thick product, allowing to keep tensile
strength properties at mid-thickness for higher tempering
parameters. On the other hand, an optimization of Vanadium and
Niobium contents reduces the softening during tempering. This
creates stable carbides known to be resistant to softening. In any
cases, the amounts of B and Nb are kept at a very low level.
700 c===~~----------------------------, .lt,
-
lnfltnnce of tempering parameter on YS
L.!J.fu!if!l_ thick o 149/nm thick 4 193mm thick " 2~~ ""
W>+----202>:{1
'"'' '"" 21000
hmperlng paramHu
Figure 14: Influence of the thickness and the tempering
parameter on Yield Strength for grade 2JACr-1Mo-%V.
Figure 14 above describes the evolution of the yield strength as
a function of the tempering parameter. Even if the trend is
obviously the same as for UTS, it appears that the margin is larger
regarding the minimum requirement to be met.
Anyway these two last figures clearly show the influence of
multiple P\VHT on the mechanical properties of the base metal. The
chemical analysis optimization has given some extra margin but the
material is close to its metallurgical limit.
Figure 15 shows the average values of Charpy-V impact testing
obtained at -l8C (for Q&T) and -29C after some P\VHT. Results
concern plate thickness between 127 and 230mm with quarter
thickness and half thickness sampling. It can be noticed that the
impact properties remain completely satisfactory for all the
considered LMP range, whatever the thennal history of the plate.
Thus toughness does not appear to be a limiting factor for the time
being in tem1s of acceptable PWHT.
Averagl) CharpyVva!ue (18'C Q&T; 29'C PWHT) r..-mmm thick. o
14Srnm thick _. tnmm thick < 23~
m 1n:: -~1:"""-m~:--' i' ~,---- ~ E ~ "' i . -t, ''t=-----1,, ~
3""t----! i -'' . p ' ,, i ~ ! g IS'J
r------------r-"--~->i-~--~' ~----~ --t ~~: w ~ ~ [J
toll , t. ~ ----- ... M --== .. -=-=-======~ moo 21200
Figure 15: lnfluencc of the thickness and the tempering
parameter on impact properties for grade 2%Cr-1 Mo-%V.
To summarize, the optimization carried out on 2'.4Cr-1Mo-'4V
steel has allowed to reach the level of mechanical properties
required for 250 mm thick plates while respecting a LMP at
21200.
CONCLUSIONS Modern steels have to be considered with care
when
addressing the way pressure vessels fabrication will be
performed, in particular the associated heat treatments such as
PWHT. The need to make service conditions safer and safer has lead
to stringent specifications for steel supply. The best compromise
must then be found between materials properties and heat treatments
needed for welds. Too high post weld heat treatment will
deteriorate the base metal properties and consequently decrease the
service performance of the vessels. Some improvements can be made
but always targeting the whole behaviour of the equipment. This can
be done through adaptation of chemical composition, especially for
CrMo V steels, or manufacturing route as in the case of C-Mn where
quench and temper can bring solutions for very thick products. To
save some PWHT margin, it is also possible to perform the tempering
treatment at a lower temperature than the PWHT and this solution is
more and more used today in the industry to match both properties
and manufacturing constraint at the same time, with no adverse
effect on the final properties of the parent metal nor the
weld.
REFERENCES [I) Jaffe L.D., Buffum D.C., "Upper-Nose
Embrittlement in
Ni-Cr Steel", Trans. AIME, 1957. [2] Libsch, J.F., Powers A.E.,
Bhat G, IVfemper Embrittlement
in Plain Carbon Steels", Trans. ASM, Vo1.44, pp.1058-1075,
1952.
[3] Pellini \V.S., Queneau B.R., "Development of Temper
Ernbrittlement in Alloy Steel", Trans. ASM, Vol.39, pp.l36-161,
1947.
8 Copyright 2010 by ASME
