OPTIMISATION OF ISR AND PWHT OF CrMoV STEELS

OPTIMISATION DES TRAITEMENTS THERMIQUES INTERMEDIAIRES ET DU DETENSIONNMENT DES ACIERS CrMoV

S. PILLOT, P. BALLADON, P. BOURGES, INDUSTEEL

A. BERTONI, AIR LIQUIDE WELDING - ETC

M. CLERGE, C. BOUCHER, INSTITUT DE SOUDURE

RESUMÉ

Durant la fabrication et la réparation en service, les appareils à pression sont soumis à différents traitements thermiques intermédiaires (ISR) et traitements thermiques après soudage (PWHT) selon les impositions des codes de construction.

Dans certains cas les impositions sont apparues trop restrictives (ISR au lieu d’un post-chauffage DHT) ou au contraire trop imprécises (températures inappropriées)

La détermination de traitements optimisés dépend de l’objet du traitement et de ses conséquences sur le dégazage de l’hydrogène, l’adoucissement et la relaxation des contraintes dans les matériaux concernés.

Cette optimisation a été faite sur l’acier 13CrMoV9.10. Les critères sélectionnés sont les propriétés mécaniques (traction et résilience), la déshydrogénation et la relaxation.

La simulation numérique a permis de déterminer le comportement de l’hydrogène et l’évolution des contraintes résiduelles.

Des conditions optimisées par rapport aux recommandations usuelles sont proposées.

ABSTRACT

During fabrication and repair in service, pressure vessels are submitted to various Intermediate Stress Relieving (ISR) and Post Weld Heat Treatments (PWHT) following requirements proposed by construction codes.

In some cases these requirements have appeared too stringent (ISR in place of Dehydrogenation Treatment DHT) or on the contrary too imprecise (too low temperatures admitted).

The determination of optimised Heat Treatment conditions depends on the purpose of this treatment and therefore on the dehydrogenation, the tempering and the stress relieving behaviour of the materials to be concerned.

This optimisation has been done on 13CrMoV9.10 materials. The chosen criteria are the mechanical properties (tensile and CVN) and the Dehydrogenation and Stress Relieving effects.

Besides mechanical characterisation, numerical simulation has been used for the determination of hydrogen behaviour and the evolution of residual stresses.

The optimised conditions are compared to the requirements of the usual construction codes and buyer’s requirements.

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INTRODUCTION

During pressure vessel fabrication, some hydrogen can be introduced in welds through the weld deposit. Besides residual stresses are generated by the local heat treatment due to welding, involving local expansion and contraction.

Heavy pressure vessels are particularly sensitive to these problems due to the additional restraint associated to high thickness. In consequence they have to be heat treated to reduce hydrogen content and/or residual stresses. Two types of heat treatments are used to solve the previous problems.

Dehydrogenation Heat treatment (DHT) or sometimes Post-Heating is done at low temperature (typically below 400°C) to insure the diffusion of hydrogen outside the sensitive areas.

Intermediate Stress Relieving (ISR) is done at mid temperatures (typically between 600°C and 680°C) to insure a partial removal of the residual stresses in the weld.

At the end of fabrication a Post Weld Heat Treatment (PWHT) is carried out to define properly the service properties of the structure.

POSITION OF THE PROBLEM

Some years ago, some workers had shown the strong interest to perform DHT in substitution to ISR in the case of standard 2.25Cr1Mo (EN 10028-2 10CrMo9.10) steels1.

But during fabrication of reactors in ASTM A542 D steel (similar to EN10028-2 13CrMoV9.10), some problems have appeared dealing with cracks initiated in the welded zones, propagating in the base material2.

The ISR conditions that are used for the usual EN10028-2 12CrMo9-10 had proved to be inefficient for EN10028-2 13CrMoV9.10 steel.

To assess the effects of these intermediate heat treatments, a collaborative study was performed to characterise:

- The mechanical properties of the different weld zones for the various heat treatments;

- The diffusion of hydrogen in the welds;

- The evolution of the residual stresses for the various intermediate heat treatments.

All these elements permit to propose some ways to optimise the Heat Treatment conditions for 13CrMoV9-10 steel pressure vessels.

MATERIALS

The materials are presented on table 1. All the materials are typical of the high quality that can be obtained by an accurate control of steel making (base metals and wires) and of minerals (fluxes and coatings).

C Mn P S Si Cu Ni Cr Mo Al Nb V Sb Sn AsBM 0.135 0.57 0.004 0.002 0.070 0.060 0.080 2,25 1.05 0.009 0.021 0.272 0.002 0.005 0.003SA 0.100 0.80 0.007 0.005 0.180 0.030 0.090 2.48 1.07 - 0.021 0.260 0.001 0.003 0.003

SMA 0.085 0.86 0.007 0.005 0.210 0.030 0.080 2.32 1.08 - 0.021 0.270 0.002 0.004 0.004Table 1: Chemical composition typical values (%) [SA: Submerged Arc Welding; SMA: Shield Metal Arc Welding]

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BEHAVIOUR OF MATERIALS DURING AND AFTER WELDING

Base material

The mechanical properties of the base material are strongly dependant from the tempering conditions. It appears on figure 1 that the yield strength limits given by the European Standard imply a PWHT not higher than about 21000.

300

400

500

600

700

19500 19700 19900 20100 20300 20500 20700 20900 21100 21300 21500

LM Pr = T*[20+logt] (K-h)

RT 450°C

Figure 1: Yield Strength of base material

Figure 2 illustrates the evolution of toughness in the base material with tempering. The NDT goes below 0°C only for tempering higher than 19500 despite very high values of CVN. It can be noticed that these results are in accordance with the usual correlations between CVN and NDT3.

-160

-140

-120

-100

-80

-60

-40

-20

0

20

17500 18000 18500 19000 19500 20000 20500 21000 21500

LM Pr = T*[20+logt] (K-h)

TK54J NDT

Figure 2: CVN Transition Temperature Tk54J and NDT of base material

1 E. Takahashi, K. Iwai, “Omission of Intermediate Post Weld Heat treatment (PWHT) by Utilizing Low-Temperature PWHT for Welds in Pressure Vessels”, ASTM STP 755, p.4182 L.P. Antalffy, G.T. West, “The new generation vanadium modified reactor steels”, EFC WP 15 meeting minutes, Paris La Défense, 15 November 20023 G. Sanz, “La rupture des aciers 1 : La rupture fragile”, Collection IRSID-OTUA, Septembre 1974

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Heat affected Zone

The heat-affected zone has two main evolutions to take in account: hardness and toughness.

It is clear that the actual HAZ is strongly tempered by the multipass thermal cycles. Heat treatments lower than 19300 have no effect. This means that the tempering effect of welding can be estimated to this slightly high value of the parameter. This is in accordance with some assumptions done for numerical simulation of Weld Metal properties4.

Weld Metal

Figure 6 illustrates the softening of weld metals during heat treatments. A 20300 LMP minimum is required insuring hardness below 250.

DISCUSSION

As observed before1 the main problems appear in the weld metal during fabrication.

CONCLUSION

The assessment of the occurrence of the various thermal treatments that can be done during and after fabrication has been performed for the 13CrMoV9.10 grade.

4 Ph. Bourges, L. Jubin, P. Bocquet, “Prediction of Mechanical Properties of Weld Metal based on some Metallurgical Assumptions” in “Mathematical Modelling of Weld Phenomena”, The Institute of Material, 1993, edited by H. Cerjak and K.E. Easterling

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