Post-weld stress relieving (SR), also known as post-weld heat treatment (PWHT), is frequently required for critical components made of S460QL and similar quenched and tempered steels. The reason lies in the conflict between the welding process, which imposes a severe local thermal cycle, and the sophisticated quenched and tempered microstructure of the base metal. Stress relieving corrects three damaging effects of welding and restores the joint to a tough, stable condition.
S460QL is a quenched and tempered high-strength structural steel with a minimum yield strength of 460 MPa. Welding subjects the base metal to intense localized heating and rapid cooling, which creates residual stresses, transforms the heat-affected zone (HAZ) into hard brittle phases, and introduces diffusible hydrogen. Each of these effects must be addressed for the weldment to perform as intended.
Residual Stress Relief
During welding, the weld metal and HAZ contract as they cool but are restrained by the surrounding cold base metal, generating high tensile residual stresses that can approach the yield strength of the material. These locked-in stresses add to the applied service loads, reducing static capacity, shortening fatigue life, and increasing susceptibility to stress corrosion cracking. Stress relieving heats the component to a temperature typically between 550 and 600 deg C, which is below the original tempering temperature of about 620 deg C for S460QL. At this temperature the yield strength is temporarily lowered, allowing the locked-in elastic strains to relax through localized plastic deformation. Slow controlled cooling then leaves the component in a more uniform, low-magnitude stress state.
Tempering the Heat-Affected Zone
The welding thermal cycle produces a gradient of microstructures in the HAZ. The coarse-grained region closest to the weld is heated to very high temperatures, causing grain growth, and on rapid cooling it can transform into hard, brittle untempered martensite. This is the most crack-sensitive part of the weld and a primary initiation site for fractures. The SR thermal cycle acts as a global tempering treatment: it tempers the hard martensite into softer, more ductile tempered martensite, dramatically improving the toughness and ductility of the entire HAZ.
Hydrogen Removal and Prevention of Delayed Cracking
Even with ultra-low-hydrogen consumables and strict preheat, welding can introduce atomic hydrogen into the weld metal and HAZ. This hydrogen diffuses and accumulates in regions of high tri-axial stress, which can lead to hydrogen-induced or delayed cracking occurring hours or days after welding, after the component has passed initial inspection. Heating the component to the SR temperature greatly accelerates hydrogen diffusion; holding at temperature for a sufficient time, for example about 2 hours per 25 mm of thickness, allows the hydrogen to escape, effectively eliminating the risk of delayed cracking.
When Is SR Required?
Stress relieving is generally required, or strongly recommended, for:
Thick sections, typically above about 30-35 mm, as specified by design standards or fabricator specifications.
Complex, highly restrained weldments such as nodes and intersections.
Components subject to dynamic or fatigue loading, such as cranes and offshore structures.
Components for low-temperature service where toughness is critical.
Components that will undergo subsequent precision machining, where residual stress redistribution could cause distortion.
Typical SR Parameters for S460QL
Heating rate: slow, typically a maximum of about 150 deg C per hour, to avoid creating new thermal stresses.
Soaking temperature: typically 580 deg C plus or minus 15 deg C, and critically at least 25-30 deg C below the original tempering temperature of the base metal to avoid softening and loss of specified properties.
Soaking time: dependent on thickness, for example about 2 hours per 25 mm of thickness.
Cooling rate: controlled slow cooling inside the furnace, typically a maximum of about 200 deg C per hour down to about 300 deg C, after which air cooling is acceptable.
The exact cycle is defined in a qualified procedure and verified by a procedure qualification record.
If stress relieving is skipped on a critical thick-section S460QL weldment, the component may enter service with a high risk of brittle fracture under dynamic or impact loading, severely reduced fatigue strength, a risk of delayed hydrogen cracking, and dimensional instability as residual stresses redistribute during machining or service.
Frequently Asked Questions
What is stress relieving (SR)? Stress relieving, also called post-weld heat treatment, is a controlled heating and cooling cycle applied to a welded component to relax residual stresses, temper the heat-affected zone, and remove diffusible hydrogen.
Is SR mandatory for all S460QL welds? No. It is required for thick sections, highly restrained joints, fatigue-loaded or low-temperature components, and parts needing precision machining. For thin, lightly restrained welds, the design code and the qualified welding procedure determine whether SR is needed.
What temperature is used for SR of S460QL? Typically about 580 deg C plus or minus 15 deg C, which must stay below the original tempering temperature of about 620 deg C to avoid softening the base metal.
How long should the component be held at the SR temperature? The holding time depends on thickness, typically about 2 hours per 25 mm of thickness, as defined in the qualified procedure.
Why must the SR temperature stay below the tempering temperature? Exceeding the original tempering temperature would further soften the quenched and tempered base metal and reduce its specified yield and tensile strength.
What happens if SR is omitted? The weldment may suffer brittle fracture under dynamic loading, reduced fatigue strength, delayed hydrogen cracking, and dimensional instability caused by residual stress redistribution.