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Research Papers: Materials and Fabrication

Potential Detrimental Consequences of Excessive PWHT on Pressure Vessel Steel Properties

[+] Author and Article Information
Cédric Chauvy

ArcelorMittal Industeel, Chateauneuf, F42800, Rive de Gier, FranceCedric.chauvy@arcelormittal.com

Lionel Coudreuse

ArcelorMittal Industeel, Chateauneuf, F42800, Rive de Gier, FranceLionel.coudreuse@arcelormittal.com

Patrick Toussaint

ArcelorMittal Industeel, 266 Rue de Chatelet, B6030 Charleroi, Belgium e-mail: Patrick.toussaint@arcelormittal.com

J. Pressure Vessel Technol 134(2), 021401 (Jan 11, 2012) (6 pages) doi:10.1115/1.4005054 History: Received February 21, 2011; Revised August 25, 2011; Accepted August 30, 2011; Published January 11, 2012; Online January 11, 2012

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 production heat treatment of the base material, which leads to achieve the target properties, is most of the time followed by post weld heat treatment (PWHT). The aim of such treatments is to insure a good behavior 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 unacceptable results. In some cases for fabrication purposes an intermediate Stress relieving treatment can be required. Special attention is paid on C-Mn steels (e.g., SA/A516 from ASME BPV Code) with the effect of thickness and Ceq (International Institute of Welding Carbon equivalent formula: see page 3) requirements on the final compromise between properties and heat treatments. In particular, toughness and ultimate tensile strength (UTS) are the critical parameters that will limit the acceptance of too high PWHT. Although micro-alloying is a mean to increase the resistance to PWHT, this leads to difficulties in softening the heat affected zones. This solution is therefore not the best one considering the whole equipment optimization. Finally, the manufacturing process can play a major role when specifications are stringent. Quenching and tempering (Q&T) 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, PWHT 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/A387gr11 is very typical of metallurgical changes that can occur during these high PWHT 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 (705 °C and even 710 °C) because of welds toughness issues. Optimization 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 Larson–Miller parameter (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 behavior of the equipment as a whole. In particular, the tempering operation can sometimes be done at lower temperature than PWHT in order to offer the best properties to the final vessel.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Influence of temperature and time on tensile properties (grade A387gr22 cl2)

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Figure 2

Mechanical properties as a function of LMP (grade A387gr22 cl2)

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Figure 3

Influence of tempering parameter on UTS values for A516gr70

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Figure 4

Influence of increasing thickness on Charpy values for Normalized C-Mn steels at given LMP value

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Figure 5

Influence of PWHT on hardness values for A516gr70 without micro-alloying elements

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Figure 6

Influence of PWHT on hardness values for C-Mn steel containing micro-alloying elements

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Figure 7

Evolution of mechanical properties for A516gr70 (Ceq 0.43%) as a function of heat treatment and LMP

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Figure 8

Evolution of CVN toughness at 0°C for A516gr70 (Ceq 0.43%) as a function of heat treatment and LMP

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Figure 9

Evolution of tensile properties of A387gde11cl2 as a function of the tempering parameter

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Figure 10

Evolution of toughness (Kv −29 °C) of A387gde11cl2 as a function of the tempering parameter

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Figure 11

Influence of tempering parameter on tensile strength of 2¼Cr-1Mo steel

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Figure 12

Influence of alloying on UTS for 2¼Cr-1Mo-¼V (thk  >150 mm, properties at ½ thk): Curves representing minimum values

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Figure 13

Influence of LMP and thickness on UTS for the grade 2¼Cr-1Mo-¼V

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Figure 14

Influence of the thickness and the tempering parameter on Yield Strength for grade 2¼Cr-1Mo-¼V

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Figure 15

Influence of the thickness and the tempering parameter on impact properties for grade 2¼Cr-1Mo-¼V

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