Abstract

It is known that Stress Relaxation Cracking (SRC) has occurred in welded joints or cold deformation areas and is a severe problem for some units in hydro-treating or hydro-cracking processes.

For example, in NACE (1) Paper No.07423 from CORROSION 2007, TNO Science and Industry investigated the SRC ranking of heat resistant alloys and the mechanism of SRC within a Joint Industrial Programme (JIP). They clarified that the major mechanism is due to the formation of both precipitation hardening along dislocations and precipitation free zone along M23C6 carbide at grain boundaries during stress relaxation process in the temperature range between 550°C and 750°C.

Stainless alloys S34709 or S30409 indicate higher SRC susceptibility compared with S31609, and requires Post Weld Heat Treatment (PWHT) to prevent SRC. Therefore, end users and fabricators using S34709 or S30409 have a strong interest austenitic stainless steels with superior SRC resistance where PWHT can be eliminated.

In our research, proprietary version S34751 was studied due to its extreme low carbon and higher nitrogen contents. Both stress relaxation testing and thermodynamic calculations revealed that proprietary version S34751 had higher SRC resistance than S34709 and S30409 without PWHT.

Introduction

Various austenitic stainless steels such as UNS S30409, S31609, S32109 and 34709 are widely used in complex refinery or chemical plants at temperature ranges between 550°C and 950°C. However, Stress Relaxation Cracking (SRC) in welded joints or cold deformed parts has been a serious problem during fabrication or operation. Several researches were conducted to construct SRC test methods. This included the evaluation of SRC susceptibilities among various austenitic stainless steels and to determine SRC mechanism within TNO Science and Industry or JIP1-4. It was concluded that SRC was caused by the accommodation of strain due to both carbide/nitride precipitation hardening inhibiting dislocation movement and the formation of precipitation free zone along the M23C6 carbide at grain boundary during stress relaxation process of welding residual stresses at temperatures between 550°C and 750°C.

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