Nowadays welding is the most common way to connect metal parts and structures. One of the challenges connected to welding it that heat output from the welding alters the microstructure of the metal creating the heat affected zone (HAZ) near the weld. In steel welds HAZ is often harder and more brittle than the base material due to formation of martensite. This might cause hydrogen induced cracking and speed up the fatigue of the weld. To mitigate the martensite formation in the HAZ different heat treatments, like preheat, interpass and PWHT are often applied. However, for 4130 steel, preheat and interpass temperatures are not expected to restrict martensite formation due to materials slow transformation rate. Preheat and interpass temperatures are still important for hydrogen diffusion and reduction of tension in the weld.

This paper investigates the effect of different heat treatments on the microstructure of AISI 4130 steel used in sour service pipes. The welding and sample preparation were performed in accordance with ISO 15156 and ASME B31.3 standards. Two sample sets were produced: one with and one without preheating. The hardness tests of weld profiles were performed in accordance with ISO 15156-2 international standard. Comparison of hardness profiles indicated that preheat had virtually no effect on hardness of the steel in HAZ, although it affected hardness of fusion zone. Preheated samples were further heat treated in a furnace simulating PWHT effect. Three different PWHT condition were tested. The hardness profiles indicated that PWHT led to noticeable changes in steel microstructure. In order to understand those microstructure changes, the heat treatment of the steel during production process was reviewed and microscopic investigations of the weld profiles were performed.

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