Low alloy steels are used extensively for high temperature applications in power plants and the petrochemical industries but, while they have good weldability, they also present distinct challenges when they have been aged through service. This is because these steels, when service aged, are particularly susceptible to embrittlement and cracking.
Repair welding generally requires the use of a relatively high preheat, good interpass temperature control, and prolonged Post Weld Heat Treatment (PWHT). However, PWHT is difficult to perform in an in-service industrial environment, meaning that several welding procedures that do not require this process have been developed. These procedures are often referred to as ‘temper bead’ repairs.
While these non-PHWT procedures are permitted by many Codes and Standards the application of these techniques is not widespread. Furthermore, these non-PHWT procedures are not currently allowed by Codes and Standards for the repair of vanadium modified grades, which are claimed to be better-suited for high temperature and hydrogen service.
Service Life Issues
A major problem associated with repair welding of service removed components is the possibility of temper embrittlement. Temper embrittlement refers to the decrease in notch toughness of alloy steels when heated in, or cooled slowly through, a temperature range of 400°C to 600°C. Temper embrittlement can also occur as a result of isothermal exposure in this temperature range, and is caused by the presence of impurities such as antimony, phosphorous, tin, and arsenic.
Furthermore, low alloy steel welds are potentially susceptible to hydrogen-induced stress corrosion cracking (HSCC) and hydrogen embrittlement following repair and in a range of environmental conditions. As would be expected from a hydrogen embrittlement phenomenon, material susceptibility increases with increasing hardness, and hence control of the problem in practice is based largely on the avoidance of unacceptably hard microstructures. Even though the developed temper bead repair procedure may limit the hardness within generally acceptable limits to avoid SCC, there is a possibility of localised hard regions. Significant residual stress also may be present in the repaired location, which could make the repair location susceptible to SCC.
It remains to be determined how temper bead repair welds perform when compared with more conventional repair methods.
Despite the possibility that non-PHWT methods of repair can offer acceptable microstructure and adequate mechanical properties where they are permitted, there are still concerns over service life when applied in an industrial environment. Repaired locations can show significant residual stresses, which can have a detrimental effect on service life, including fatigue and corrosion fatigue performance in the repaired region.
Currently there is limited quantified data on the performance of both types of repair in typical service environments, which means that selecting the most appropriate repair route can be challenging.
Additional data on the comparison between different repair methods on service aged Cr-Mo low alloy steel in a variety of service environments could be used to identify the most appropriate repair method, but would also improve the confidence in the integrity of a repair weld, and could save both time and money.
Further research could also assess the feasibility of using a temper bead repair for vanadium modified Cr-Mo steel, check how susceptible temper bead repair and conventional repair are to embrittlement, and assess each type of repair for their effect on service life. This testing would be particularly useful in regard to low alloy steels used in power plants and petrochemical industries.
TWI is looking to assess factors involved and to build data on the different approaches in order to optimize repair schemes for service-aged components. A proposed project outline can be accessed here.
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