Welding of Maraging Steels

Steels of the 9Ni-4Co system, including 30N8K4KhMF steel (VKS-6), are characterised by higher fracture toughness and cold resistance. Consequently, they are regarded as promising materials instead of 30KhGSN2A steels in heavily loaded structures for important applications.\" However, welding of these steels may be accompanied by a decrease of the mechanical properties of the welded joints as a result of unfavourable changes in the structure of these steels (woody fracture, insufficiently quenched areas, overaged areas, etc.). The aim of this work was to select technology resulting in the formation of welded joints in 30N8K4KhMF steel with parameters similar to those of the parent metal. Investigations were carried out on specimens with a thickness of 2-50 mm. At a small thickness, the joints were produced by automatic argon TIG welding without filler, large thicknesses were welded y electron beam welding (EBW) and by various methods of argon welding: manual and automatic narrow-gap TIG welding in argon without a filler, automatic submerged-arc welding under 48-OF6m and AN-15M fluxes, belonging to the group of passive fluxes and preventing the burn-out of carbon and alloying elements from the deposited metal. The filler wire was in the form of Sv-18N8KhMF-VI wire, developed on the basis of the examined steel. Two technological variants were investigated: with and without subsequent heat treatment. The heat treatment conditions: normalisation at 900 °C for 1 hr + quenching from 830 °C in oil + cold treatment at 7 0 ° C for 2 hr + tempering at 540 °C for 2 hr. The welded joints are subjected to the tensile test (crB), the bend test (a), the impact toughness test (KCT, KCU) with evaluation of the fracture surface of the tested specimens and the low-cycle fatigue tests with determination of to number of cycles to fracture Nc at a stress of a 735 MPa with the asymmetric cycle 'max 0' with a loading frequency of 10 cycles per minute. The test results were statistically processed. This was followed by determining the confidence range on the basis of the Student distribution for a reliability of 0.95. Other tests included evaluation of the susceptibility of welded j oints to the formation of hot (Ac ) and cold (<rc) cracks in tests in LTP1-6 and LTP2-5 equipment, as well as using an SEV-19KhT technological testpiece. The welded joints were subjected to metallographic examination with measurement of hardness and determination of the austenite content in heating Ah and cooling Ac of the parent and filler metal by the ballistic method.

Special features of welding high-strength, high-alloy steel with mixed carbide-intermetallic hardening

We determine the brittle-fracture resistance of the metal taken from various zones of welded joints of 34KhN3MA steel produced from electrodes with different phase compositions or their combinations. It is shown that, regardless of the zone of welded joint in which a notch is made in the specimen, the highest values of impact toughness are observed for the weld metal obtained from the austenitic electrode. At the same time, the lowest value of impact toughness are observed for the combination of austenitic (weld root) and ferrite-pearlite electrodes. For any applied electrode, the lowest values of impact toughness were always observed for the metal of the heat-affected zone. Therefore, this zone determines the serviceability of welded joints under dynamic loads. The results of fractographic investigations demonstrate that austenitic electrodes are optimal for the purpose of welding of sheets made of high-strength steels because they guarantee the highest fracture energy of the joints under the action of dynamic loads.

Impact Toughness of Welded Joints on 34KhN2MA Steel Produced from Electrodes with Various Phase Compositions

Structure of the metal in the fusion zone in depositing austenitic steel on pearlitic steel

Welding of 30KhGSN2A steel in the quenched and tempered condition through a transition piece of 12Kh2NVFA (EI712) steel

: Recent studies of the developments in welding steels with yield strengths greater than 150 ksi have included low-alloy martensitic steels, medium-alloy martensitic steels, nickel maraging steels, and bainitic steels. Only weldments from medium-alloy martensitic steels and nickel maraging steels have mechanical properties approaching those of the base plate without a complete postweld heat treatment. The most serious problem with the other steel is low toughness in the weld fusion zone. Adequate weld metal toughness under conditions of elastic strain can be obtarined over the entire 150 to 225 ksi yield-strength range only if the tungsten-arc welding process is used. Processes with higher deposition rates can produce comparable weld deposits only in the lower portion of the range. Above a yield strength of 200 ksi, 18Ni maraging steel weldments have the best combination of strength and toughness. Below 200 ksi, the HP 9-4-25 medium-alloy martensitic steel and 12Ni maraging steel weldments have nearly equal properties.

D. C. Martin

Papers

Welding high-strength steels,