Thermo-Calc used to Estimate Critical Temperatures in Type 410 Steels

In the paper Alloy Composition and Critical Temperatures in Type 410 Steel Welds, Thermo-Calc was used to evaluate the effect of alloy composition on critical temperatures (A₁ and A₃) in Type 410 steels.

Type 410 steels are typically welded by using consumables with matching composition. However, this type of steel has shown to have poor weldability which is related to formation of hard and brittle martensite in the weld zone, hydrogen-included cracking or retention of δ-ferrite which affects the toughness. One theory that explains the inconsistent toughness is that the wide composition ranges of the base metal results in wide variations of the A₁-temperature. In the paper, this theory was investigated with the design of experiment (DoE) approach using Thermo-Calc to perform thermodynamic simulations. Thermo-Calc together with the TCFE8 database was used to predict A₁ and A₃ temperatures for various compositions.

An A1 temperature predictive diagram was plotted based on the A1 temperature predictive formula and chromium and nickel equivalent equations. The diagram was validated by comparison to published data and through phase transformation analysis of alloy within the AWS and ASTM compositional specification ranges. The predictive accuracy of the A1 temperature formula, calculated by Thermo-Calc, proved to be within the error of experimental temperature measurement (±3°C).

The paper is written by D. J. Stone, B. T. Alexandrov and J. A. Penso at the Ohio state university.

Abstract

The design of experiment (DoE) approach using thermodynamic simulations with ThermoCalc™ was applied to evaluate the effect of alloy composition on the critical temperatures in Type 410 steels and welding consumables. A predictive equation and predictive diagram for the A1 temperature were developed and verified through experimentation and comparison with published data. This was also complemented with the development of a predictive equation for the A3 temperature.

The results of this study show that the combined ASTM and American Welding Society (AWS) compositional specifications for Type 410 materials provide a range of A1 temperatures that is significantly wider than the postweld heat treatment (PWHT) temperature range specified by the American Society of Mechanical Engineers (ASME). This creates a potential risk of exceeding the A1 temperature during PWHT, resulting in formation of fresh martensite, and can be related to difficulties meeting hardness and toughness requirements for Type 410 welds experienced in industry. Narrowing the ASTM and AWS compositional specifications by introduction of lower limits for all alloying elements, including nitrogen and copper, was identified as a potential solution to this problem.

The predictive tools developed in this study can be applied for selection of welding consumables and base metals, postweld heat treatment (PWHT) temperature selection, and compositional optimization of Type 410 steels and welding consumables.