LA SELECTION DES CONDITIONS POUR LE TRAITEMENTTHERMIQUE APRES LE SOUDAGE DES CORDONS DE SOUDURE

MIXTE

THE POST WELD HEA T TREA TMENT OF DISSIMILAR MA TERIALS

I. DetempleK. RichterA. Kolling

Dillinger HüttenwerkeDillinger HüttenwerkeDillinger Hüttenwerke

RESUME

Un recuit de detente doit etre effectuer Sill des elements soudes pour plusieurs raisons. Lesauteurs de specifications ne tiennent souvent pas compte du fait que les proprietes mecaniquesdu materiau de base diminuent en fonction de l' augmentation de la temperature du recuit dedetente et/ou bien de la duree de maintien. De par ce fait il se peut que le temps alloue pour lerecuit de detente soit tellement eleve dans les specifications que les proprietes macaniques ettechnologiques du materiaux de base tombent en dessous des valeurs minimales defmies.

La situation se complique encore lorsque differents types de metaux sont soudes dans unelement de construction commun et doivent sub ir par la suite un recuit de detente. Pourcomprendre leg difficultes dans le choix des conditions de recuit de detente pour differentsmetaux, il est necessaire de tenir compte de l'influence du recuit Sill leg proprietes mecaniquesdes aciers appliques.

ABSTRACT

For a number of reasons, it is customary tor post weid heat treatment (PWHT) to beperformed on welded steel structures. Specification writers are often not aware of the factthaI the mechanical properties of the base material will be weakened with increasing stressrelief temperatures and/or holding tim es. Therefore the situation might occur where thespecification prescriptions tor PWHT are so intensitive thaI the mechanical properties of thebase material can slip below the minimum requirements of the specification.

The situation becomes more complicated when different materials are welded together in onecomponent and afterwards stress relieved. To understand the difficulties in choosing the rightPWHT

conditions tor diss im ilar joints, it is necessary to take the effect of PWHT treatment onthe mechanical properties of the different steel qualities info account.

INTRODUCTION

1

For a number of reasons, it is customary für post-weId treatments (PWHT) to be performed onwelded steel structures. The main objectives are as foliows:

...

Reducing the residual stresses ofthe weldedjointsSoftening of hard and brittle structuresIncreasing the toughness of the weId metal

Specification writers are orten not aware of the fact that the mechanical properties of the basematerial will be weakened with increasing stress relief temperatures and/or holding times.Therefore, a situation might occur where the specification prescriptions für PWHT are sointensive that the mechanical properties of the base material can slip below the minimumrequirements of the specification.

The situation becomes even more complicated when different materials are welded together ina component, and afterwards stress relieved. In a recently published article [1] the authorsuggested what post-weld-heat temperatures should be used für joined dissimilar materials ina very simplified war. His answer was in principle that, " iftemperatures ranges für PWHT

of the two materials overlap, star in the overlapping range. If both temperature ranges justreach each other, take the temperature of the material with the higher temperature range, hutas low as possible ..."

These recommendations seem to be very logical hut they cannot be applied in every case, asthey do not consider the real behaviour ofthe "weaker" material ofthe dissimilar joint.

To understand this restriction, it is necessary to give same general information on the effect ofPWHT treatment on the mechanical properties of steel quality, and on the concept of theHollomon Parameter, provided in the first part of this article [2]. The possibilities of PWHTconsidering dissimilar materials will be covered in the second part

THE HOLLOMON -JAFFE PARAMETER

The Hollomon Parameter (HP) combines the PWHT temperature and holding time in onlyone value and is expressed by the equation:

10-3HP = T .(20 + lOglO t)

where T is the holding temperature of the heat treatment in Kelvin and t the holding time,given in hours.

Furthermore, it is possible to combine several PWHT treatments in only one value and also touse only one HP value für tempering and PWHT. This is demonstrated infigure 1. This figureshows that:

.

Different heat treatments with the same HP have the same metallurgical effect on thematerialThe higher the HP, the higher the metallurgical effect on the material

.2

18 18,5 19 19,5 20 20,5 21

HP. Pa_ter

Measured variation ofRm as a function ofHP for steel grade A387-11-2, N+" in various temperingand/or PWHT conditions i

,

Fig.l:

18 18,5 19 19,5 20 20,S 21HP. Parameter

Measured variation of Charpy-V-transverse toughness, testing temperature -18 °C, as a function ofHPfür steel grade A387-11-2, N+T, in various tempering and/or PWHT conditions

Fig.2:

F or only nomlalized- and nomlalized plus tempered materials, increasing HP values lead todecreasing Rm, Rpo,2 and impactvalues. An example ofthe decrease ofRm ofanASTM A387-11-2 quality is given in figure 1. A further example for the variation of the Charpy- V impactvalues in dependence ofthe HP-factor for the same quality infigure 2.

PRODUCTION WINDOW FOR A SINGLE STEEL GRADE

As mentionned above, the mechanical properties of a steel grade will be reduced withincreasing HP values. Furthermore, most standards define ranges für the ultimate tensilestrength (R"J and toughness (Ch- V) at a given testing-temperature, leading to a limitation ofthe applicable HP values.

3

Figure 3 gives an example of this. The left hand part schematically shows the variation of Rmwith its 99 % confidence limits versus HP for a given chemical analysis and a plate thicknessof 10 mm, together with the upper and lower Rm limits for this quality. As can be seen fromthis figure, it is necessary to apply a minimum HP of 18.6 to avoid the risk ofgetting too highRm values, and a maximum HP of 20.3 to avoid too low Rm values. The HP-range between18.6 and 20.3 define the production-window 1 for Rm.

m"50

:2-

~

18 18,4 18,8 19,2 19,6Hollornon-Parameler

20 20,418 18,4 18,8 19,2 19,6 20 20,4

Influence of scattering mechanical properties on the estimation of a reliable production window fot aconstant plate thickness of 10 mm.

Fig.3:

In the fight hand part of figure 3, the toughness with its lower 99 % confidence limit at agiven testing temperature is plotted versus HP. It becomes clear, thai a maximum HP of 19.55can be applied to avoid too low Ch- V values, leading to the production window 2. To makethe things less complicated, it is assumed thai the minimum yield strength value (Rpo.2J isuncritical, i.e. can be reached under all HP conditions.

Now, to determine what would be the common production window für the 10 mm thick plateof this quality, both production windows have to be combined and the overlapping range ofboth will be the real production-window:

Für Rm:Für Ch-V:Für RpO,2:

18.6 ~ HP ~ 20.318.6 ~ HP ~ 19.5518.6 ~ HP ~ 20.3

...

The common HP to fulfill all specified requirements is situated between 18.6 and 19.55. Ifthis method is applied to other thicknesses, this will lead to the results as shown infigure 4.The production-window für Rm (dark grey area) and toughness (light grey area) respectively,are plotted against the thickness für this quality. This common production- window is markedby the hatched area.

4

Tensile and Toughness Toughness .Tensile

20.5

20.0

19.5

19.0

18.5

18.0

Graphical representation of combinations of plate thickness and HP fOT which a given set ofmechanical requirements (ten.vile and toughne.v.v) is fulfilled with acceptable risk

Fig.4:

This hatched field can now answer the question of which PWHT conditions are applicable. Arealistic calculation ofthe production-window für an A387-11-2 with a given analysis in thedelivery condition N+T (T =680 °C/30 minutes) with a tougness requirement of 21 (17) J at 0°C has been performed, the result is shown infigure 5.

The differences ofthe HP-values between the line ofHP=18.8 (which reflects the temperingcondition), and the upper limit of the light grey area define the maximum allowable PWHT-condition for the given chemical analysis and heat treatment, to fulfil the minimum andmaximum prescribed mechanical properties, respectively.

For same plate thicknesses, the length ofthe arrows infigure 5 represent the maximum LlliPwhich can be used für PWHT. The labels beside the arrows ','translate" the shown AHP into arealistic PWHT condition at a temperature of 690 °C with variable holding times. For lowertemperatures the holding times would naturally be longer and vice versa.

In the next chapter, we will discuss the subject "PWHT of dissimilar joints" by combining theproduction-windows of different steel grades.

5

Toughness Tensile

Fig.5: Graphical representation of the production window für steel grade A387-11-2 in norrnalized andtempered condition, für a given chemical composition, für which RpO.2 ~ 311 MPa, 515 ~ Rm ~ 690MPa and Av (O°C) 17 (21) J is fulfilled

PWHT CF DISSIMILAR MATERIALS

Following the above given explanation considering the PWHT possibilities for singlematerials, the answer which stress relief condition would be possible for dissimilar materialswelded together will be discussed with an example of avessei, made of dissimilar materialsaccording European standards. The requirements on the mechanical properties of those steelgrades depend on the plate thickness leading to very complex production windows.

~~

"/

P 355 NHN

e = 55 mm

-~

P 355 NHN

e = 55 mm,

/'"

~~

/' "

~/1

\ I/

~'",

Figure 6:

13CrMo 44N+A

e = 95 mm

Schematical representation of avessei with dissimilar materials

6

15Mo3N

= 70 mm

We assume a reactor as shown schematically infigure 6. It consists ofthree mall parts suchas foliows:

The vessel welded together is supposed to be post-weld-heat treated at 670 °C/4 hours. Wewill check now, whether this PWHT is realistic or not. The filled region infigure 7 shows thecombinations of thickness and HP where all mechanical properties of the steel grade13CrM044 with the given chemical analysis are fulfilled with an acceptable risk.

Figure 7: Vessel of 13CrMo44 with its production window considering PWHT conditions as a functionof the plate thickness

The tooth-shaped curve is the result of the stepwise decrease of the minimum tensile strengthwith increasing thickness according the European material standards. The delivery conditionN+T leads to an HP-Value of 18.36 which is marked in the diagramm with a line. In theabove mentionned example, we consider a plate thickness of 95 mm. The length of the plottedarrow in figure 7 defines the parameters für the maximal possible PWHT condition, which isgiven in this case with 670 °C/620 minutes.

As described above, flgure 8 defmes the combinations of HP and thickness für the steel grade15M03 (for the given chemical composition) used für the main flange, which are possible toreach the requirements ofthe code. Since the delivery condition is "normalized" and the platethickness is assumed to be 70 mm, the maximal PWHT condition is here 670 °C/960 minutes.

7

0 20 40 60

Thlckness [mm]

80 100

Figure 8: Main flange of 15Mo3 with its production window considering PWHT conditions as a functionof the plate thickness

The four small flanges were produced with steel grade P355NH with a chemical compositionshown in figure 9. Applying the same method as described above, the components with aplate thickness of 55 mm only endure PWHT at 670 °C of only 10 minutes.

20 40 60

Thlckness [mm]

80 100

Small flange of P355NH with its production window considering PWHT conditions as afunction of the plate thickness

Figure 9:

8

Taking all used steel grades into ac count, a PWHT at 670 °C/4 hours is not possible withoutfailing the purposed mechanical properties. An initial stage für a solution of this problem is tochoose other steel grades für the four small flanges.

CONCLUSION

Any kind of steel grades are defined by a special frame of the chemical composition andrequirements according minimum- resp. maximum values of yield strength, tensile strengthand toughness. Since the performed heat treatment, especially PWHT, can influence thesemechanical properties so intensively that they can slip below the minimum requirements ofthe specification.

The heat treatment parameters, such as holding temperature and -tinle can be expressed by asingle value, the so-called Hollomon parameter (HP). It is also possible to express sequencesof PWHT by a single HP. Using this tool, HP ranges can be determined for any steel grade asa function ofthe plate thickness in which a given PWHT can be performed uncritically.

When applying this tool to dissimilar materials, it might be done the following:

1.

Detemlination of the production window for each material seperately as describedabove.Determination of the common production window of all involved steel grades byoverlapping the single production windows (HP, plate-thickness).

2.

This procedure gives the opportunity to estimate in advance, whether a planned PWHTcondition is applicable on a welded joint without running the risk of failing the mechanicalproperties of the weakest steel grade in the joint. In this context it should be mentioned thaithe chemical composition, the plate thickness and the heat treatment condition has to be takeninto account for the above proposed procedure.

Unfortunately, it is not possible to give very general guidelines to solve this problem. It istherefore recommended to involve the steel producer at an early stage in the conception of thePWHT condition.

REFERENCES

G. E. GNIRSS, "Post-weId heat treatments", Welding in the world/Soudage dans lemonde, VoI. 39, 1998, pp. 243-247

[1]

I. DETEMPLE and A. DEMMERATH, "The Effect 0/ Beat Treatment", HydrocarbonEngineering, November 1998, p. 67

[2]

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