Yasuhiro Nishio

Weld crack susceptibility of duplex stainless steel. (Report VI). Effects of nitrogen and molybdenum on weld solidification crack susceptibility.

Weld solidification crack susceptibility and pitting corrosion resistance of tentative duplex stainless steels were investigated in order to decrease the crack susceptibility without decreasing the corrosion resistance in a duplex stainless steel. The results obtained are as follows:(1) Crack susceptibility was improved with decreasing the N content in 22 to 25Cr-1 to 10Ni duplex stainless steels, but was not considerably changed with increasing the Mo content up to 4.5% at constant level of 0.03%N.(2) Corrosion resistance was improved with increasing the Mo content at constant level of 0.03%N.As a result, it was suggested that a lower N and Mo in the range of 4.0 to 4.5% were very effective for the development of a duplex stainless steel which had the low crack susceptibility without decreasing the corrosion resistance.

Studies on Surface Modification of Nonferrous Metals by means of Plasma Ion Nitriding (PIN) Process (Report 4). Microstructure and Hardening Mechanism of Nitrided Ni Binary Alloys.

In order to investigate the hardening mechanism of Ni alloys by nitriding treatment, tentative Ni binary alloys of Ni-7wt%Ti, Ni-15wt%V, Ni-10wt%Nb and Ni-30wt%Cr were intrided by plasma ion nitriding (PIN) process under various conditions of nitriding temperature from 673 to 1073K and nitriding time at 10, 8ks in N2+H2 mixed gas atmosphere of 800Pa.Hardness of the nitrided layer was increased with decrease of nitriding temperature. From the X-ray diffraction analysis and transmission electron microscopy, it was considered that the hardening mechansim of Ni alloys by PIN process was due mainly to the lattice strain of the matrix of nitrided layer caused by dispersive precipitation of metastable nitrides of each alloying element. A lower hardness of the nitrided layer at higher nitriding temperature was owing to the decrease of lattice strain by coarsening of equilibrium nitride particles.

Cold cracking susceptibility in weld metal of HY-steel. Part 5. Relaxation phenomenon of the restraint stress due to transformation expansion in weld zone of HY-steel.

This study was done to develop the possibility of sound welding of higher grade HY-type of steel having a low lower critical stress, utilizing transformation expansion of weldment including weld metal and the heat-affected zone.The restraint stresses in root pass welded joint of various strength steels such as SM41, HT60, HT80, HY130 and HY150 were quantitatively evaluated utilizing the RRC test under the restraint intensity of 9.8 and 29.4 kN/mm.mm by GTA welding with filler wire. On the other hand, transformation expansion of welded joint was measured by dilatometric technique. Then the relationship between the restraint stress and the transformation expansion was quantitatively investigated.Consequently, the final value of the restraint stress reduced with an increase in strength of base steel, namely in the order of SM41, HT60, HT80, HY130 and HY150 under the same restraint intensity. For example, the final restraint stress in SM41, HY130 and HY150 was about 490, 216 and 147 MPa, respectively, in the restraint intensity of 9.8 kN/mm·mm. Reducing the restraint stress was attributed to transformation expansion of welded joint. The increase of transformation expansion was much effective for reducing the final restraint stress in HY130 and HY150 weldments in comparison with the other weldments. The transformation temperatures of HY130 and HY150 weldments were lower than those of other weldments, so that not only large transformation expansion but also low transformation temperature were very useful to reduce the final restraint stress. In addition, these results suggested that the crackfree weldments of HY130 and HY150 steels may be completed by developing the higher alloy filler wire having high expansion coefficient and low transformation temperature during cooling after welding.

Study on the Improvement of Bending Properties of 405 Stainless Steel Weldment (Report 1)

Bending result of 405 stainless steel weldment is remarkably affected by chemical composition of welded materials, especially Al content, as described in previous report. Various Al content specimens are used in this experiment, and their bending test results, mechanical properties of bond are studied.Effects of Veining structure, hydrogen content of weld metal are also researched.Results are as follows:1) Bending properties of 405 stainless weldment are remarkably affected by Al content of welded materials.2) Welded joint of 405 stainless steel which contains more Al and less C is poor in bending properties, and low temperature post heat treatment is necessary for improving the bending properties.3) Veining structure and hydrogen in weld metal have no effect on bending properties.

Study on Welding Crack of 405 Stainless Clad Steel

AISI 405 stainless steel and its clad steel are widely used in chemical apparatus, such as oil refiners, heat exchangers and so on.Several cracks are often found in the weldment with the same chemical composition electrode (D41ONb) after welding or annealing. Investigations on phenomena of these cracking and their prevention were carried out.Conclusions are as follows.1) Delayed cracking with very long incubation period occurs in 405 stainless steel weldment; this was never experienced in the structural steel weldment.2) These cracks are prevented by preheating and adoption of MIG welding. Reducing the hydrogen content of deposited metal is most necessary to preverrt these cracks.3) Cracking occurs in high Cr content weldment such as Mr, 17Cr, and even in almost ferrite structure. These phenomena are very different from those in structural steel.