The subject of the work discussed herein and carried out as a part of the Polish National Centre of Research and Development project titled ”Development and implementation of critical technology demonstrators for the new generation of 120 mm tank artillery ammunition” are the results of a research into the influence of LPS (Liquid Phase Sintering) parameters and heat treatment on the mechanical properties of W91Ni6Co3 and W91Ni6Co2.25Fe0.75 alloys, designated PR200 and PR201, respectively. The alloys, as LPS-processed and heat treated, were tested on a strength testing machine to determine their tensile strength (Rm), proof stress (Rp0.2) and elongation (A5). The analysis of the test results resulted in a proposal of manufacturing process parameters to have the alloys tested develop specified mechanical properties. It was found the ternary alloy with chemical composition W91Ni6Co3 and designated PR200 was more promising in the feasibility of producing specified mechanical properties. The alloy, once sintered and heat treated in two stages, could facilitate production of a material with a tensile strength Rm>1400 MPa, a yield strength Rp0.2 > 1350 MPa, a minimum elongation of 11%, and an impact strength > 115 J/cm2.

The aim of this study is to determine the susceptibility to hydrogen embrittlement in X37CrMoV5-1 steel with two different microstructures: a nanocrystalline carbide-free bainite and tempered martensite. The nanobainitic structure was obtained by austempering at the bainitic transformation zone. It was found, that after hydrogen charging, both kinds of microstructure exhibit increased yield strength and strong decrease in ductility. It has been however shown that the resistance to hydrogen embrittlement of X37CrMoV5-1 steel with nanobainitic structure is higher as compared to the tempered martensite. After hydrogen charging the ductility of austempered steel is slightly higher than in case of quenched and tempered (Q&T) steel. This effect was interpreted as a result of phase composition formed after different heat treatments.Keywords: carbide-free bainite, tool steel, austempering, hydrogen embrittlement Celem pracy było określenie wrażliwości na kruchość wodorową stali X37CrMoV5-1 o dwóch różnych mikrostrukturach: nanokrystalicznego bainitu bezwęglikowego i odpuszczonego martenzytu. Struktura nanobainityczna została otrzymana w wyniku procesu hartowania izotermicznego w zakresie przemiany bainitycznej. Zaobserwowano, że po procesie katodowego nasycania wodorem oba typy mikrostruktury wykazują zwiększenie granicy plastyczności i znaczący spadek plastyczności. Wykazano jednak, że odporność na kruchość wodorową stali X37CrMoV5-1o strukturze nanobainitycznej jest lepsza w porównaniu do stali o strukturze martenzytu odpuszczonego. Po katodowym nasycaniu wodorem plastyczność stali hartowanej izotermicznie jest nieznacznie wyższa niż w przypadku stali hartowanej i odpuszczanej (Q&T). Zjawisko to zostało zinterpretowane jako wynik składu fazowego wytworzonego podczas różnych obróbek cieplnych.

Tungsten heavy alloys (THA) are used in the defense industry for subcaliber bullet cores due to their high density and strength. Typically methods of joining tungsten rod elements include: soldering, friction welding or threaded sleeve splicing. The properties of the joints were tested for three types of material containing 90.8, 96.2 and 98.2 wt.%. tungsten, density from 17.3 to 18.4 g/cm3 and strength range 400–1000 MPa. Combination in the liquid phase at the sintering temperature was carried out in a vacuum furnace at a temperature of 1520 °C in a hydrogen atmosphere, and tests used pairs of both identical and dissimilar materials. After that, some of the bars were subjected to additional heat treatment at 1100 °C for 3 h. The tests of the mechanical properties in the static tensile test and the measurement of impact strength showed that the obtained strength of the joints was comparable to that of the parent material. The microstructure analysis showed that the resulting joint area, while maintaining the appropriate roughness of the joined end faces of the bars, is homogeneous without areas of the solidified matrix of the joint line. Research showed that it is possible to bond under sintering conditions with the participation of a solid liquid phase of homonymous and dissimilar THA materials. The strength of joints in dissimilar materials was comparable to a tungsten heavy alloy material with lower strength in the bonded pair while homonymous materials were comparable to the original material. The test results provided a good basis for further research in which the obtained pairs of joints will be subjected to plastic working processes.

The results of studies of W-Ni-Co-Fe experimental alloy, with chemical composition assuring a possibility of producing Ni-based supersaturated solid solution are presented. The alloy was prepared from tungsten, nickel, cobalt and iron powders which were first mixed then melted in a ceramic crucible where they slowly solidified in hydrogen atmosphere. Next specimens were cut from the casting and heated at a temperature 950 o C. After solution treatment the specimens were water quenched and then aged for 20 h at a temperature 300 o C. The specimens were subjected to microhardness measurements and structure investigations. The latter included both conventional metallography and SEM observations. Moreover, for some specimens X-ray diffractometry studies and TEM investigations were conducted. It was concluded that quenching lead to an increase of tungsten concentration in nickel matrix which was confirmed by Ni lattice parameter increase. Aging of supersaturated solid solution caused strengthening of the Ni-based matrix, which was proved by hardness measurements. The TEM observation did not yield explicit proofs that the precipitation process could be responsible for strengthening of the alloy.

This paper presents a multi-objective optimization method for optimizing the process parameters during friction welding of dissimilar metals. The proposed method combines the response surface methodology (RSM) with an a genetic algorithm (GA) method. Ultimate tensile strength (UTS), flash diameter and the heat affected zone (HAZ) width of friction welded nodular cast iron with low carbon steel joints were investigated considering the following process parameters: friction pressure (FP), friction time (FT) and upsetting pressure (UP). Mathematical models were developed and the responses were adequately predicted. Direct and interaction effects of process parameters on responses were studied by plotting graphs. In the case of UTS, FT has high significance followed by: FP and UP. Friction time has high significance on the flash diameter of nodular cast iron followed by UP and FP. However in the case of the low carbon steel flash diameter, UP has high significance followed by FT and FP. In the case of the HAZ width for nodular cast iron and low carbon steel side, friction time has high significance followed by UP and FT. Multiobjective optimization for maximizing the tensile strength and minimizing the flash diameter and the HAZ width was carried out using mathematical model.