Zeron 100 superduplex stainless steel was first developed in the 1980s and it was invented with a tightly controlled melt chemistry which together with a restricted heat treatment temperature range ensured the optimum combination of mechanical properties, corrosion resistance, weldability and hot working characteristics. When the alloy was included in ASTM, ASME, BS, EN, NACE and other standards, Zeron 100 data was submitted whilst the alloy was eventually listed under the UNS S32760 designation. UNS S32760, however, has a much wider range of chemical composition than Zeron 100. During the 1990s this grade of superduplex stainless steel became very popular with the oil and gas industry for both seawater and process systems. This commercial demand has resulted in the S32760 grade being offered by an increasing number of alloy producers. However, the pressure to minimize production costs has resulted in some manufacturers pushing the composition to the limits permitted by the UNS specification. This can give unusual combinations of analysis and shows a lack of fundamental understanding of the required control in composition and thermo-mechanical processing.

For example, nickel levels have been reduced and nitrogen contents have been increased, together with hot working and final heat treatment disciplines that are variable. These variables are not directly apparent from material certification. There are many examples of the problems that this causes, where commercial heats of S32760 exhibit worse than expected properties. The paper describes a series of laboratory tests on several commercial heats of S32760 to illustrate this point.

One effect which can result is the precipitation of nitrides. In an attempt to understand the effects of the poor control on precipitation behavior an arbitrary nitrides scale was developed which assigned a number from 0 to 3 based on an examination of a large number of fields on a microsection. Evaluation of impact toughness showed a sharp reduction in toughness with increasing nitride content. Corrosion tests also showed a substantial reduction in corrosion resistance with increasing nitride content.

These effects are discussed in the context of industry using material which does not meet the properties that were submitted originally to enter the grade into international standards.