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Welding of Martenistic Stainless Steels |
Welding of Martenistic Stainless Steels - During welding, when martensitic steels are heated (to about 927°C) austenite forms. Rapid cooling of the metal transforms the high-temperature austenite to martensite, thereby introducing transformation stresses. Moreover, because of low thermal conductivities of these steels, sharp heat gradients (large temperature difference across small areas) are produced during welding.
The unequal thermal expansion caused by these gradients plus volumetric changes (internal expansion produced by transformation of austenite into martensite) create stresses that can cause cracking, especially in some of the brittle microstructures formed/by rapid cooling rate.
To reduce the welding and transformation stresses and to increase the toughness and ductility of the weld deposit, the following procedure may be used:
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Preheating: Before welding, the material is to be preheated between 149 and 260°C to prevent cracking. Post weld heating, if required, should be carried out immediately after welding.
The most common post weld treatment consists of heating the weldment to 732 to 788°C and holding at that temperature for about one hour per 25 mm of thickness. Cooling should be slow, at a rate of about 10 degrees C per hour, to 593°C. Below this point the weldment is cooled in air. 3
The preheat will reduce the severity of the thermal cycle and the post weld heat treatment will temper the martensite that is formed.
If preheating and post heating treatments are not possible, austenitic type electrodes should be employed because they produce welds that are less crack-sensitive.
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