STRUCTURE AND PROPERTIES OF LASER-BEAM-WELDED JOINTS OF LOW-ALLOY HIGH-STRENGTH STEEL DOCOL 1200M WITH A MARTENSITIC STRUCTURE

This article presents studies on the effects of the linear energy of the laser-beam welding of the DOCOL 1200M high-strength low-alloy steel with a martensitic structure, 1.8-mm thick, on the properties and structural changes of the joint area. The welding process was carried out in the horizontal position, without a filler material, at a variable linear energy of the welding ranging from 25 J/mm to 55 J/mm. The non-destructive testing allowed the joints to be classified as the quality level B in accordance with ISO 13919-1. In order to determine the structural changes and changes in the properties, the joints were subjected to destructive testing including macro- and microscopic metallographic examinations, a hardness measurement, a static bend test with the face and root of the weld in tension and a static tensile test. The destructive testing showed that with an increase in the linear energy of the welding, the plastic properties of the joints are enhanced while, at the same time, their strength properties are reduced to the values below that of the base material. The results showed that with the use of the lowest linear energies of laser-beam welding (around 25 J/mm), it is possible to produce welded joints of the DOCOL 1200M steel, whose strength is equal to that of the base material, which is not achievable when arc welding the steels with such a high strength (1200 MPa). The joints made with the liner energy of 25 J/mm were characterised by a tensile strength of about 1240 MPa, obtaining a bending angle of 70 degrees. The weld hardness was about 440 HV1 and it was similar to that of the base material while in HAZ, the hardness decreased to 360 HV1.