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One trend in today’s structural design for automobiles is towards application of stainless Steel in addition to traditional carbon Steel. This is mainly due to the advantages offered by stainless steel to the designer, i.e. its high corrosion resistance, heat resistance, strength and formability. Welding of austenitic stainless steel involves a high risk of solidification cracking due to low melting eutectics containing different kinds of impurities and to accumulated strain behind the weld pooi in the mushy zone. As a result, the quality of stainless steel welded joints will greatly be affected which may give cause for safety concerns. The intention of this study was to investigate the hot cracking sensitivity of laser welded fully austenitic stainless steels and to determine the critical local strain and strain rate from the formation of solidification cracks. C02-laser welding experiments were conducted using the steel X8CrMnNi19-6-3 (Nirosta H400). Solidification cracking susceptibility was examined with the help of the Controlled Tensile Weldability (CTW) Test developed by BAM Federal Institute for Materials Research and Testing in Berlin. The critical strain and strain rate of solidification cracking was measured at the surface of the workpiece directed to the laser beam in the close vicinity of the weld pool by using the Digital Image Correlation (DIC) technique and diode laser as the illuminating source.
Due to significant developments over the last decades, laser beam welding has become a well-established industrial process offering high processing speeds and causing low component distortions. But an important issue currently preventing its intense use, especially in the energy or plant construction sector where high alloy steels are applied, concerns hot crack formation. Although considerable advances in understanding hot cracking mechanisms have been made, most of the known influencing factors are metallurgical in character. The thermo-mechanical effects are barely considered or quantified. Up to the present, there exist numerous hot cracking tests that were however conceived for welding methods other than laser beam welding. Considering the special features of the laser welding process, such as high cooling rates and the narrow process zone, results obtained with other welding techniques and test procedures cannot be transferred to laser beam welding. In this study, the laser beam weldability of various stainless steels was examined in terms of their susceptibility to hot cracking by means of the controlled tensile weldability test, which was proven to be suitable for use in conjunction with CO2 laser welding. This test allows the application of tensile strain at a variable fixed cross-head speed transverse to the welding direction. Full and partial penetration bead-on-plate welds were produced. In a first attempt to determine the impact of the applied external strain on the local transient strains and strain rates near the weld pool, an optical system was used to measure the backside surface of partial penetration welds. The results showed the influence of the strain and the strain rates on hot crack formation. Furthermore, a classification of the studied austenitic, duplex and ferritic stainless steels according to the established test criteria (critical strain and cross-head speed) was conducted.