Kiyoshi Kiuchi

Hot cracking behaviour and susceptibility of extra high purity type 310 stainless steels

Abstract Recent progress in the refining technology has enabled the production of highly pure commercial stainless steels. The hot cracking behaviour of these stainless steels was investigated with respect to type 310 stainless steel. For comparison, four types of 310 stainless steels with various amounts of minor and impurity elements such as C, P and S were used. The purity of type 310 stainless steels used was enhanced in the order of type 310<type 310S<type 310ULC<type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse Varestraint test. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that in type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of type 310EHP steel was sufficiently low as not to yield ductility-dip cracking in practical multipass welding. The experimental and numerical analyses on the solidification brittle temperature range have revealed that the reduced solidification cracking susceptibility upon decreasing the amount of C, P and S in stainless steel can be attributed to the reduced BTR due to the suppression of minor and impurity elements, such as C, P and S, in the finally solidified liquid film between the dendrite.

Quantitative influence of minor and impurity elements on solidification cracking susceptibility of extra high purity type 310 stainless steel

Abstract To evaluate the influence of minor and impurity elements on the solidification cracking susceptibility in austenitic stainless steel welds, the transverse Varestraint test was conducted using several extra high purity type 310 stainless steels with different amounts of C, Mn, P and S. The characteristic influence on solidification cracking could be expressed by the ratio of P/S/C≈1∶1·3∶0·56, while Mn affected solidification cracking to a negligible extent. A theoretical approach revealed that the reduced solidification cracking susceptibility with decreasing C, P and S contents could be attributed to the suppression of solidification segregation.

Influence of Minor and Impurity Elements on Hot Cracking Susceptibility of Extra High-Purity Type 310 Stainless Steels

The hot cracking behaviour of extra high-purity stainless steels was investigated with respect to type 310 stainless steel with various amounts of minor and impurity elements such as C, P, S and Mn. The purity of the type 310 stainless steels used was enhanced in the order of Type 310<Type 310S<Type 310ULC<Type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse-Varestraint test. This test revealed that two types of hot cracks occurred in these steels; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that Type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of Type 310EHP steel was sufficiently as low as not to yield ductility-dip cracking in practical welding. Numerical analysis suggested that the reduced solidification cracking susceptibility upon refining C, P and S could be attributed to the reduced solidification brittle temperature range due to the suppression of solidification segregation of minor and impurity elements. The quantitative contribution of minor and impurity elements to the hot cracking susceptibility of extra high-purity type 310 stainless steels was evaluated by using lab-melted steels with different amounts of C, P, S and Mn. The essential influence on solidification cracking was the ratio of P:S:C=1:1.3:0.5, while Mn negligibly ameliorated solidification cracking in the extra low S (and P) steels. On the other hand, a molecular orbital analysis to estimate the binding strength of the grain boundary suggested that the increased ductility-dip cracking susceptibility in extra high-purity steels was caused by grain boundary embrittlement due to the refining of beneficial elements for grain boundary strengthening such as C.

Influence of Impurities on Intergranular Corrosion of Extra High Purity Austenitic Stainless Steels

An intergranular corrosion is observed in austenitic stainless steels exposed to high temperature, concentrated nitric acid (HNO3 ) solution with highly oxidizing ions. It is an important degradation mechanism of austenitic stainless steels for use in a nuclear fuel reprocessing plant. The intergranular corrosion is caused by the segregation of impurities to grain boundaries and the resultant formation of active sites. Extra High Purity (EHP™) austenitic stainless steel was developed with conducting the new multiple refined melting in order to suppress the total harmful impurities less than 100ppm. The intergranular corrosion behavior of EHP alloys with various impurities was examined in boiling HNO3 solution with highly oxidizing ions to find a correlation between the intergranular corrosion and the impurities of EHP alloys. A good correlation was confirmed between the degree of intergranular corrosion and the corrosion rate. The relationships between the corrosion rate and the impurities content of EHP alloys was determined using a multiple regression analysis. The influence on corrosion rate became small in order of B, P, Si, C, S and Mn. It was important to control B in intergranular corrosion behavior of EHP alloys.Copyright

Susceptibility of Intergranular Corrosion for Extra High Purity Austenitic Stainless Steel in Nitric Acid

Austenitic stainless steels suffer intergranular attack in boiling nitric acid with oxidants. The intergranular corrosion is mainly caused by the segregation of impurities to grain. An extra high purity austenitic stainless steel (EHP alloys) was developed with conducting the new multiple refined melting technique in order to suppress the total harmful impurities less than 100ppm. The basically corrosion behavior of type 310 EHP alloy with respect to nitric acid solution with highly oxidizing ions was investigated. The straining, aging and recrystallizing (SAR) treated type 310 EHP alloy showed superior corrosion resistance for intergranular attack. The segregated boron along the grain boundaries was one of main factor of intergranular corrosion from fission track etching results. The SAR treatment was effective to restrain the intergranular attack for type 310 EHP alloy with B less than 7ppm.Copyright