The investigations were inspired with the problem of cracking of steel castings during the production process. A single mechanism

of decohesion – the intergranular one – occurs in the case of hot cracking, while a variety of structural factors is decisive for hot cracking

initiation, depending on chemical composition of the cast steel. The low-carbon and low-alloyed steel castings crack due to the presence

of the type II sulphides, the cause of cracking of the high-carbon tool cast steels is the net of secondary cementite and/or ledeburite

precipitated along the boundaries of solidified grains. Also the brittle phosphor and carbide eutectics precipitated in the final stage

solidification are responsible for cracking of castings made of Hadfield steel. The examination of mechanical properties at 1050°C

revealed low or very low strength of high-carbon cast steels.

In the high-alloy, ferritic - austenitic (duplex) stainless steels high tendency to cracking, mainly hot-is induced by micro segregation

processes and change of crystallization mechanism in its final stage. The article is a continuation of the problems presented in earlier

papers [1 - 4]. In the range of high temperature cracking appear one mechanism a decohesion - intergranular however, depending on the

chemical composition of the steel, various structural factors decide of the occurrence of hot cracking. The low-carbon and low-alloy cast

steel casting hot cracking cause are type II sulphide, in high carbon tool cast steel secondary cementite mesh and / or ledeburite segregated

at the grain solidified grains boundaries, in the case of Hadfield steel phosphorus - carbide eutectic, which carrier is iron-manganese and

low solubility of phosphorus in high manganese matrix. In duplex cast steel the additional factor increasing the risk of cracking it is very

"rich" chemical composition and related with it processes of precipitation of many secondary phases.

In the paper, verification of welding process parameters of overlap joints of aluminium alloys EN AW-6082 and EN AW-7075, determined on the grounds of a numerical FEM model and a mathematical model, is presented. A model was prepared in order to determine the range of process parameters, for that the risk of hot crack occurrence during welding the material with limited weldability (EN AW-7075) would be minimum and the joints will meet the quality criteria. Results of metallographic and mechanical examinations of overlap welded joints are presented. Indicated are different destruction mechanisms of overlap and butt joints, as well as significant differences in their tensile strength: 110 to 135 MPa for overlap joints and 258 MPa on average for butt joints.

Nickel alloys, despite their good strength properties at high temperature, are characterized by limited weldability due to their susceptibility to hot cracking. So far, theories describing the causes of hot cracking have focused on the presence of impurities in the form of sulphur and phosphorus. These elements form low-melting eutectic mixtures that cause discontinuities, most frequently along solid solution grain boundaries, under the influence of welding deformations. Progress in metallurgy has effectively reduced the presence of sulphur and phosphorus compounds in the material, however, the phenomenon of hot cracking continues to be the main problem during the welding of nickel-based alloys. It was determined that nickel-based alloys, including Inconel 617, show a tendency towards hot cracking within the high-temperature brittleness range (HTBR). There is no information on any structural changes occurring in the HTBR. Moreover, the literature indicates no correlations between material-related factors connected with structural changes and the amount of energy delivered into the material during welding.

This article presents identification of correlations between these factors contributes to the exploration of the mechanism of hot cracking in solid-solution strengthened alloys with an addition of cobalt (e.g. Inconel 617). The article was ended with development of hot cracking model for Ni-Cr-Mo-Co alloys.