OPTIMISATION OF ISR AND PWHT OF CrMoV STEELS
OPTIMISATION DES TRAITEMENTS THERMIQUES T HERMIQUES 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 différe différents nts traiteme traitements nts thermiqu thermiques es intermé intermédiai diaires res (ISR) (ISR) et traiteme traitements nts thermiqu thermiques es après après soudage (PWHT) selon les impositions des codes de construction. Dans certains cas les impositions sont apparues trop restrictives (ISR au lieu d’un postchauffage DHT) ou au contraire trop imprécises (températures inappropriées) La déter détermin minat ation ion de trait traiteme ement ntss optimi optimisés sés dépend dépend de l’obje l’objett du trait traiteme ement nt et de ses ses consé conséque quence ncess sur le dégaz dégazage age de l’hyd l’hydrog rogène ène,, l’adou l’adouci cisse ssemen mentt et la relax relaxati ation on 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 simul simulat ation ion numér numériqu iquee a permis permis de déter détermin miner er le compo comporte rtemen mentt de l’hyd l’hydrog rogène ène et l’évolution des contraintes résiduelles. Des conditions optimisées par rapport aux recommandations usuelles sont proposées. ABSTRACT
Duri During ng fabric fabricat atio ion n and and repa repair ir in serv servic ice, e, pres pressu sure re vess vessel elss are are subm submit itted ted to vari variou ouss Interm Intermedia ediate te Stress Stress Reliev Relieving ing (ISR) (ISR) and Post Post Weld Weld Heat Heat Treatm Treatment entss (PWHT) (PWHT) follow following ing requirements proposed by construction codes. In some ome case casess thes thesee requ requir irem emen ents ts hav have app appeare eared d too too stri string ngen entt (ISR (ISR in place lace 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 treatme treatment nt and theref therefore ore on the dehydr dehydroge ogenat nation ion,, the temper tempering ing and the stress stress reliev relieving ing behaviour of the materials to be concerned. This optimisation has been done on 13CrMoV9.10 materials. The chosen criteria are the mechan mechanical ical proper propertie tiess (tensi (tensile le and CVN) CVN) and the Dehydr Dehydroge ogenat nation ion and Stress Stress Reliev Relieving ing effects. Besi Beside dess mech mechan anic ical al char charac acte teri risa sati tion on,, nume numeri rica call simu simula lati tion on has has been been used used for for the 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. 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) steels 1. 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 material 2. 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
0.13 0.5 5 7 0.10 0.8 SA 0 0 0.8 SMA 0.08 5 6 Table 1: Chemical Welding] BM
P
S
Si
0.00 0.00 0.07 4 2 0 0.00 0.00 0.18 7 5 0 0.00 0.00 0.21 7 5 0 composition typical
Cu
0.06 0 0.03 0 0.03 0 values
Ni
Cr
Mo
Al
0.08 2,2 1.0 0.00 0 5 5 9 0.09 2.4 1.0 0 8 7 0.08 2.3 1.0 0 2 8 (%) [SA: Submerged Arc
Nb
V
Sb
Sn
As
0.02 0.27 0.00 0.00 0.003 1 2 2 5 0.02 0.26 0.00 0.00 0.003 1 0 1 3 0.02 0.27 0.00 0.00 0.004 1 0 2 4 Welding; SMA: Shield Metal Arc
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. RT
450°C
700 600 500 400 300 19500
19700
19900
20100
20300
20500
20700
20900
21100
21300
21500
LM Pr = T*[20+logt] (K-h)
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.
TK54J
NDT
20 0 -20 -40 -60 -80 -100 -120 -140 -160 17500
18000
18500
19000
19500
20000
20500
21000
21500
LM Pr = T*[20+logt] (K-h)
Figure 2: CVN Transition Temperature Tk 54J and NDT of base material 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 properties 4. 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 before 1 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.
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.418 2
L.P. Antalffy, G.T. West, “The new generation vanadium modified reactor steels”, EFC WP 15 meeting minutes, Paris La Défense, 15 November 2002 3 4
G. Sanz, “La rupture des aciers 1 : La rupture fragile”, Collection IRSID-OTUA, Septembre 1974
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