Material for May 7th 2010

Welding of Heat-Resisting Steels

Dr. Jippei Suzuki

1. Introduction

• Fig.H1

　Many types of steels or super-alloys are used for the service at elevated temperature,

depending to the temperature level and the industrial field as shown in Fig.H1. In the order

of service temperature, the steels can be arranged; mild steel, 0.5%Mo steel, 1 1/4%Cr-

1/2%Mo steel, 2 1/4%Cr-1%Mo steel, 5%Cr-1/2%Mo steel, 7%Cr-1/2%Mo steel, 9%Cr-

1%Mo steel, stainless steel and super-alloys. These heat resisting steels can be divided into

three groups; Cr-Mo steels, stainless steels and super-alloys. The temperature range, at

which the steels can be used, is dependent on the purpose of the usage.

2. Creep phenomenon

• Fig.H2

　 In general, the metal is deformed due to the applied load. The amount of deformation is

expressed by the term of strain and the magnitude of load by the term of stress. These

concepts enable to evaluate the resistance of the material against the force, independing to

the size of the material.

　 Figure H2 illustrats three stress-strain curves of a steel obtained by the tensile test,

performed at room temperature and elevated temperatures of T1, T2 (T1 < T2). At room

temperature, we can find the yield stress (point Y), the tensile strength (point B) and the

phenomena of work hardening and the necking. The yield and tensile strengthes are reduced

with a rising temperature. Therefore, the stress applied to the steel must be designed to be

lower than the strength at the elevated service temperature.

• Fig.H3

　When the steel is loaded by the lower stress than the yield stress (or the proof stress

of 0.2%) at the elevated temperature, for example, point C, temperature T1 in Fig.H2, it

brings about the elastic strain (εe). If the stress and the temperature are held, the strain

is increasing with the holding time, as shown in Fig.H3. The deformation occurs without

the increase in stress. This type of deformation is named the creep. In Fig.H3, three creep

curves are shown under three conditions. The creep curve is shifted to left-upper side, when

the stress or the temperature is increased. In reverse, the curve is shifted ro right-lower side

with a decreasing stress or temperature.

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　 Just after applying load, the steel is deformed correponding to applied stress. This

deformation is called the instantaneous extension (if the stress is tensile). After that, the

creep begins.

　 The creep rate is expressed in term of the strain rate. The strain rate is defined as the

amount of strain per unit time, and is indicated by the tangent to the creep curve. The

creep deformation is divided into three regions according to the behavior of strain rate.

1. In the early period of loading, the strain rate is relatively larger. This period is called

primary or transient creep.

2. In the middle period, the strain rate is kept constant. This period is called secondary

or steady-rate creep.

3. In the last period of loading, the strain rate is incresed and becomes very large just

before the final fracture. This period is called tertiary or accelerating creep.

• Fig.H4

　There are two types of the creep strength shown in Fig.H4. The creep tests are performed

with a variety of loading at a given temperature. And the strain rates are measured, then

a curve is obtained from some data, which represent the relation between stress and strain

rate (left). Using this curve, the stress which gives a specified strain rate can be read (point

A −→ point B).

　We can draw the curve representing the relation between stress and fracture time (right

figure). Using this curve, the stress which gives a specified fracture time can be read (point

C −→ point D).

• Fig.H5

　 Figure H5 shows the creep strength which gives the creep rate of 1%/36000ks of some

Cr-Mo steels. The time of 36000ks is about one year. The influence of alloying elements are

as followings;

– Molybdenum increases creep strength.

– Chromium increases creep strength when its content is below 1%.

– Chromium of more than 1% is added to increase resistance to oxidization.

– Chromium decreases creep strength when its content exceeds 5%.

– Molybdenum, vanadium and niobium are added for high chromium steels to increase

creep strength.

• Fig.H6

　Fig.H6 shows the creep rupture strength of 1000 hour for some types heat resisting steels.

The creep strength of Cr-Mo steels is larger than that of carbon steel and smaller than

stainless steels. The 304 type stainless steel is austenitic type, which contain chromium of

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18 ∼ 20 mass% and nickel of 8 ∼ 10 mass%. The 316 type is also austenitic, which contain

chromium of 16 ∼ 18 mass%, nickel of 10 ∼ 14 mass% and molybdenum of 2 ∼ 3 mass%.

The use of super alloys is required for the service temperature above 800℃.

3. Resistance to oxidization

• Fig.H7

　Although the creep strength is reduced due to alloying chromium of more than 1%, more

chromium is added to heat resisting steels to improve the resistance against oxidization.

Chromium forms a stable oxide film (Cr2O) on the surface of the steel and this oxide film

prevents the further oxidization of the steel. Fig.H7 shows the constitution of the steel

surface. In the case of the chromium-free steel, the scale is composed of Fe2O3,Fe3O4, FeO

and Fe-base. With increasing the chromium content, the complex oxide of (FeO)·(Fe, Cr)2O3

is formed between FeO layer and Fe-base. And the thickness of the whole scale is reduced.

When the chromium content exceeds 15%, chromium oxide Cr2 is formed between (FeO)·(Fe,

Cr)2O3 layer and Fe-base. This oxide file is very hard and adhesive closely to Fe-base.

4. Resistance to environments

• Table H1

　 There are some types of oxidization by O2, H2O, CO2. And alminum and silicon also

form the stable oxides at elevated temperature.

Vanadium forms a oxide film which has low melting point.

　 Table H1 shows the allowable service temperature of low alloy steels considering to oxi-

dization.

　 Two types of hydrogen embrittlement must be considered (Fig.H8). Because the radius

of hydrogen atom is very small, it enables to diffuse into iron-matrix. The higher the

temperature is, the easier the diffusion is. Therefore, when the steels is used for long period

at elevated temperature, under hydrogen environment, hydrogen concentration becomes

high. The hydrogen embrittlement occurs at room temperature.

• Fig.H8

　 Hydrogen atoms can diffuse more easily along grain boundary, at which carbon atoms

segregates. These atoms react at grain boundaries and form CH4 gas bubbles. These bubbles

induce the crack along grain boundary and the steels becomes brittle. This phenomenon is

named hydrogen attack.

• Fig.H9

　Fig.H9 is Nelson curve, which shows the allowable service limits under service temperature

and hydrogen pressure. The allowable service temperature is decreased with increasing

hydrogen pressure.

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5. Welding of heat resisting steels

• Fig.H10

　 In welding of heat resisting steels, followings are taken into consideration.

1. Hardening of HAZ and Lowering ductility

2. Toughness of welds

3. Cold crack induced hydrogen in weld metal and HAZ

4. Crack caused by PWHT (reheat crack)

5. Embrittlement caused by long period heating

5.1 Hardening of HAZ and cold crack in welds of heat resisting steels

• Fig.H11

　Fig.H11 shows the Jominy testing or end-quenching test for steels. This testing methods

is carried out for alloy steels for machine purpose. After the specimen is heated to austenite

region, one end of the specimen is rapidly cooled by water jet (left). Then, the hardness

testing is carried out along the axis of the specimen. The experimental results are arranged

with the distance from the end of specimen (right). These curves are named Jominy curve.

　 The maximum hardness is given at the distance of 0mm (left end), because the cooling

rate is largest there. In ABS steels, the maximum hardness is very large, however, the

hardness is decreased with the distance. These steels have poor hardenability. In HY80

steel, the maximum hardness is smaller than that of ABS steels, however, the decrease in

hardness with the distance is small. This type of steel has good hardenability.

　We must distingish the hardenability to maximum hardness.

• Fig.H12

　 Fig.H12 shows the continuous cooling transformation diagrams for three types of Cr-Mo

steels. These CCT diagrams can be used to estimate the microstructure and the hardness

of HAZ.

• Fig.H13

　Fig.H13 shows the effect of preheating on the average (left) and maximum (right) hardness

of heat affected zone. The hardness is decreased slightly by using the preheating. As shown

in Fig.H12, the CCR diagram is prolonged along the time axis, therefore, the change in

hardness is small with increasing the cooling time.

• Fig.H14

　 Fig.H14 (upper) shows that the temperature has large effect for hydrogen to escape to

the atmosphere. Although the effect of preheating on the hardness, the preheating is useful

to prevent the cold crack as shown in Fig.H14 (lower).

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• Fig.H15

　The post weld heating also is useful to prevent the cold crack (Fig.H15).

5.2 Reheat crack in welds of heat resisting steels

• Fig.H16

The weldments made from heat resisting steels would be heated after the assembly by the

welding in order to reduce the welding residual stress (stress relief annealing). In some types

of heat resisting steels, the crack will occur along the grain boundaries in the coarsened grain

region of heat affected zone (Fig.H16). At first, this type of crack was found in the weldment

made from the high strength steels containing Ni, Cr and Mo. The crack is named SR crack

or reheat crack or PWHT crack.

　The main factors are followings.

1. alloying elements

2. impurity

3. parameters of welding and PWHT

　Alloying elements affect the sensitivity of the steels to the reheat crack. Especially, carbide

forming elements are important. The influences of these elements can be evaluated by the

indices of ∆G or PSR. It must be noted that these indedices have the coverage. ∆G can be

applied to the commertial high strength steels and. The coverage of PSR is shown in Fig.16.

　 Impulities, such as P and S, are harmful.

• Fig.H17

　 Fig.H17 shows the standard PWHT temperatures for several types of steels.

5.3 Effect of PWHT on mechanical properties of heat resisting steels

• Fig.H18

　At first, if a steel is heated at the elevated temperature, and tested, the strength at the

high temperature will be smaller than that at room temperature. In general, the strength

at elevated temperature is decreased with increasing test temperature.

　 If the steel is heated, and then cooled to room temperature, the strength at room tem-

perature will reduce, depending on the period and the temperature of heating. Fig.H18

shows this change for 2 1/4Cr-1Mo heat resisting steel. The tensile strength of heat treated

specimen is examined at room temperature.

　 In this figure, the special parameter is employed. This parameter is named Larson-Miller

parameter or tempering parameter. Tow effcts of holding time and temperature can be

evaluated on the same scale by using this parameter. The strength of a steel is decreased

with increasing parameter.

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• Fig.H19

　Creep rupture also decreases due to PWHT as shown in Fig.H19 (left). In this case, the

steel welds are tested at 823K after PWHT at various temperature. Creep rupture strength

in this figure is the stress, at which the specimen will break by creep rupture after the holding

time of 10800 ks (=3000 hour=125 days) at 823K.

　Fig.H19 (right) shows the absorbed energy obtaind by Charpy impact testing. The tough-

ness is increased with the tempering parameter and become maximum. And then it is de-

creased with the parameter. As a general nature, the tempering improves the toughness of

the quenched steel. This phenomenon is very special case, therefore, it is named the temper

embrittlement.

　Temper embrittlement

– Temper embrittlement is observed in low alloy steels containing Ni, Cr, Mo. It does

not occur in plain carbon steels.

– Temper embrittlement is remarkable in the steels with high impulity, such as P, Sb, Sn

and As.

– Fracture occures along prior-austenite grain boundary.

– Temper embrittlement is reversible. Ii is possible to recover toughness, heating over

the temperature range of embrittlement.

• Fig.H20

　Fig.H20 shows the absorbed energy at 283K of 2 1/4Cr-1Mo steel heat treated. The steel

specimens were heat treated by the special method, which is called the step cooling. Be-

cause the temper embrittlement occurs after the heating for very long time, the experiments

requires long time, several months or years. Then, the step cooling method is employed

to accelarate the progress of tempering. After the quenching of the specimen, it is heated

again just below A1 temperature for 1 hour. Next, the specimen is cooled to somewhat lower

temperature and kept for somewhat longer time. This peocedures is repeated. For this case,

the time required for heat treating is about 7 days. Fig.21 (left) shows that the toughness

is affected by (Mn+Si) and impurity X .

　 From these two factors, J factor is proposed (Fig.H20 right).

• Fig.H21

　Fig.H21 shows the toughness of many types of steels of as-welded and heat treated by step

cooling. In this figure, the numerals mean the temperature in the degrees Fahrenheit, for

example, 1650AC-1250AC means that the specimen is heated up 1650 degrees Fahrenheit

(5/9(F-32)=900℃), and then cooled in air, and heated again 1250 degrees Fahrenheit (677

℃), and then cooled in air.

6. Welding of stainless steels

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• Fig.S1

　 The stainless steels are not classified into the group of heat resisting steel in common

sense. However, they are used for the service at elevated temperature. And the stainless

steel may be welded with Cr-Mo heat resisting steels.

　 The crystalline structure of stainless steel depends on mainly chromium and nickel con-

tents. Chromium is ferrite former and nickel is austenite former. The effects of other elements

on crystalline structure are evaluated in the following chromium and nickel equivalence;

Creq = %Cr + %Mo + 1.5%Si + 0.5%Nb

Nieq = %Ni + 30%C + 0.5%Mn

　 Fig.S1 is Schaeffler (Schaffler) diagram. The letters; A, F and M are austenite, ferrite,

and martensite, respectivery.

• Fig.S2

　There are 4 probles in welding the stainless steel as follows.

1. Cold cracking in the region of left-lower side.

　 In the steel of this region, austenite transforms to martensite during welding, because

the steels Cr and Ni, which improve the hardenability of steels.

2. Hot cracking in the region of upper side

　Hot crack is easy to occur in perfect austenite structure. Hot crack can be suppressed

by including ferrite of several percentage.

3. Embrittlement in the region of right lower side

Ferritic austenite does not transforme during welding, therefore, grain coarsening can

not be reformed by heat treatment.

4. Embrittlement in the region of right upper side

　 In this region, the metallic compound; σ phase precipitates. It is hard and changes

the steel brittle.

　Welding should be performed avoiding above 4 problems as shown in Fig.S2.

********* Excercise *******************************.

1. Explain the dilution.

2. Estimate the Ni and Cr contents of weld metal, when the stainless steel of 18%Cr-8%Ni is

welded using the electrode of 2%Ni-28%Cr, assuming that the dilution is 20%.

3. We should weld the stainless steel of 0.36%C-2.0%Si-0.4%Mn-14.0%Ni-15.0%Cr, and we

want that the Ni and Cr equivalents of weld metal are 8% and 22%Cr, respectively. Deter-

mine the chemical compositions of electrode, assuming that the dilution is 20%.

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4. Estimate the Ni and Cr equivalents of weld metal, when the steels of 8%Ni-18%Cr and

0.15%C-2%Cr-1%Mo are but-welded by using the electrode of 26%Cr, assuming that the

dilution is 20%.

********* Excercise *******************************.

Appendix Reheat crack of Cr-Mo steels

• Fig.J1

　 Fig.J1 ∼ J10 show the study on the reheat cracking of Cr-Mo steels. Prof.K.Tamaki.

investigated the combined influences of alloying and imputity elements for about 20 years.

　 Fig.J1 shows the testing apparatus for simulating reheat crack. It is the implant test

machine equipped with the heating furnace for simulating PWHT. A set of the specimen is

composed of the implant made by the steel to be tested and the base metal plate made by

800MPa class high strength steel. The implant is put in the hole of the base metal plate.

The welding bead is made on the base metal plate, and the implant is undergone through

the simulated weld thermal cycle. That is, the heat affected zone is made in the implant.

After loading with the screw, the specimen set is heated up to 600℃ with the heating rate

of 200℃/hour and kept at 600℃ for 20 hours.

• Fig.J2

　Fig.J2 (left) illustrates the test results. The load applied to the implant is reduced due to

the relaxation during PWHT. If the initial stress; σAW is large, the stress during PWHT;

σPW is enough large to produce the reheat crack. On small σAW, no crack occurs after 20

hours. Performing some testing changing the initial stress; σAW, the critical value; σAW-critcan be obtained to express the sensitivity to the reheat crack.

　 Fig.J2 (right) shows the σAW-crit of Cr-Mo steels against the chromium content. The

steels are divided by molybdenum content. The crack sensitivity becomes minimum at about

1%Cr for each molybdenum level. The σAW-crit is decreased (or the sensitivity is increased)

with increasing molybdenum content.

• Fig.J3

　 Fig.J3 (left upper) shows the contour curves of σAW-crit on the diagram of Cr-Mo con-

tents. The numerals are the value of σAW-crit.

　 The chemical compositions of commercial Cr-Mo heat resisting steels and high strength

steel are shown, superimposing the contour curves of σAW-crit in Fig.J3 (left lower). The

1Cr-1/2Mo and 1 1/4Cr-1/2Mo steels are sensitivi to the reheat crack, and the 2 1/4Cr-1Mo

steel and the steels containing larger amount of chromium are insensitive.

　 The indices of ∆G and PSR are expressed by straight line on the diagram of Cr-

Mo contents. Because the criteria of crack occurrence are ∆G=(%Cr)+3.3(%Mo)=0, or

PSR =(%Cr)+2(%Mo)=0, when the steels contain no alloying elements except Cr and Mo.

The ∆G and PSR correspond the σAW-crit of 400 and 550 MPa(N/mm2), respectively.

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• Fig.J4

　Fig.J4 shows the σAW-crit of Cr-Mo steels containing smaller phosphorus. The sensitivity

to crack is decreased with reducing phosphorus content. This effect depends on the Cr-Mo

contents.

• Fig.J5

　 Fig.J5 shows the influence of phosphorus for 5 types of Cr-Mo steels. Phosphorus has

no influence for 1/2Mo steel. In the cases of other steels, the σAW-crit is rapidly decreased

when the phosphorus content exceeds the value of Pcrit. The phosphorus content should be

reduced under the Pcrit. The value of Pcrit depends on the Cr-Mo contents.

• Fig.J6

　 Fig.J6 shows the influences of manganese (left side) and sulfur (right side) for the 1Cr-

1/2Mo steel. The effect of Mn content is large and S content is small.

　 Sulfur exists in steel under two conditions; dissolved sulfur and combined sulfur as MnS,

because the chemical attraction acts between Mn and S.

• Fig.J7

　 Fig.J7 shows the results shown in Fig.J6. Although the results include 4 groups of 1Cr-

1/2Mo steel containing various amounts of Mn and S, the plots can be arranged well by

the concentration of dissolved sulfur. The σAW-crit is rapidly decreased when the dissolved

sulfur exceeds the value of (S)crit, as well as phosphorus.

• Fig.J8 ∼ J9

　Fig.J8 and Fig.J9 show the influences of vanadium and titanium, respectively. Theses ele-

ments, which are carbide former, increase the sensitivity to reheat crack of steels, depending

on the Cr-Mo contents.

• Fig.J10

　Titanium and vanadium form the carbides of TiC or V4C3, respectively, during PWHT.

Then these carbides cause the secondary hardening, and the stress relaxation is reduced or

delayed. Therefore, the sensitivity is increased.

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