In order to investigate the microstructural evolution in a nitrogen-bearing 2205 duplex stainless steels (DSS) during welding, a simulated weld thermal cycle with 5 kJ cm−1 heat input followed by exposure at 700 °C for different time intervals was performed. The microstructure of high-temperature heat affected zone (HTHAZ) developed with the thermal experience was characterized via optical metallography and transmission electron microscopy (TEM). The duplex structure with equivalent phase components was drastically destroyed by the rapid thermal cycle. In the simulated HTHAZ structure, three different morphologies of newly formed austenite were observed in the coarse-grained δ-ferrite matrix; i.e. allotriomorphic austenite, Widmanstaten autenite and intragranularly nucleated autenite. During the exposure at 700 °C, the intragranularly nucleated austenite got coarse and the Widmanstaten austenite grew progressively. TEM revealed that several variants of rod-like Cr2N were precipitated selectively at intragranular and intergranular sites. From the analyses of diffraction patterns of TEM, Kurdjumov–Sachs orientation relationship was found to describe the interface between intragranularly nucleated autenite and δ-ferrite, while Pitch–Schrader orientation relationship to describe the disposition between hexagonal Cr2N precipitates and δ-ferrite matrix.

Microstructural characterization of simulated heat affected zone in a nitrogen-containing 2205 duplex stainless steel

Nitrogen loss and effects on microstructure in multipass TIG welding of a super duplex stainless steel

A microstructure–properties relationship study in two superaustenitic stainless steels (S31254 and S32654) was carried out, following exposure at elevated temperatures for various ageing times. Due to high temperature ageing, most stainless steel grades suffer the formation of various precipitates, directly affecting their properties. The full characterization of those precipitates and the correlation with the mechanical behavior of the steels is the primary aim of this study. Samples of the steel grades studied, were exposed to isothermal heat treatments within the temperature range of 650–950 °C, for ageing times varying between 0.5 h and 3000 h, followed by water quenching at room temperature. Microstructural examination indicated the formation of four different secondary phases, sigma phase (σ), chi phase (χ), Laves phase and β-Cr2N nitride, which were characterized by transmission electron microscopy (TEM) and electron diffraction. The results obtained permitted the construction of the time–temperature–precipitation (TTP) plots. In addition, tensile and Vickers hardness testing were utilized and the modulus of toughness was calculated. The kinetics of the formation of various precipitates with increasing temperature and aging duration was also observed. It was found that various precipitates had a significant effect on all mechanical properties studied.

Phase transformations and mechanical properties in heat treated superaustenitic stainless steels

Duplex stainless steels (DSS) are well known for their higher mechanical strength and better corrosion resistance. DSS is commonly used in the marine construction, petrochemical, and chemical industries. DSS (2205) has equal amounts of α and γ phases. However, unlike the parent metal, the solidification microstructure of the fusion zone in the weld does not have nearly equal amounts of the α and γ phases. Thus the mechanical properties and corrosion resistance of DSS welds are degraded. The interpass temperature plays a vital role in achieving balanced α and γ phases, which in turn results in improved mechanical and corrosion resistance. Gas tungsten arc welding (GTAW) is employed for welding of thin sheets/plates. The GTAW process ensures small amounts of slag formation during welding, which eliminates slag crevices and sites for corrosion attack. Standard 2205 (UNS S31803) DSS sheets of 5 mm thickness, with 22.37% Cr and 5.74% Ni, were used in this study. Weld beads were produced with Zeron-100 super DSS filler wire with higher alloy content (26% Cr and 8% Ni). Argon (Ar) and helium (He) were employed as shielding gases. Heat input was maintained <1 kJ/mm. Interpass temperatures were maintained at 120°C. The ferrite number of the weld metal for the two different shielding gases was investigated. Mechanical properties of joints such as impact strength and hardness were evaluated. Microstructure evaluation was also carried out.

Effect of shielding gases on mechanical and metallurgical properties of duplex stainless-steel welds

Effect of shielding gases on microstructure and mechanical properties of super austenitic stainless steel by hybrid welding

TIG welding using different heat inputs, arc lengths and shielding gas nitrogen contents was performed. The aim was to evaluate the possibilities to avoid nitrogen losses on welding or even increase the weld metal nitrogen content and thereby improve the corrosion properties and, in the case of duplex grades, also to improve the phase balance. Three different nitrogen-alloyed duplex grades and one superaustenitic grade were investigated. The corrosion resistance in terms of CPT, Critical Pitting Temperatue, of the super duplex material was found to be strongly correlated to the nitrogen content of the weld metal. In the case of the superaustenitic weld metal, the increased nitrogen content was found to be associated with an increased pore formation, leading to a lower corrosion resistance and thereby masking the positive effect of the increased introgen content. In order to illuminate the nitrogen exchange reactions between the arc, weld pool, and shielding gas, stationary weld experiments were also performed. The results from these stationary trials indicated that the net weld pool nitrogen content could be qualitatively understood if the various fluxes involved in the nitrogen transport between the plasma arc, weld pool and weld pool-shielding gas were considered. At short times the weld pool was limited in area and the nitrogen content of the weld pool increased due to high nitrogen activity in the arc. At longer times the nitrogen escaped from the weld pool to the shielding gas. This flux became then the dominating factor due to the increased weld pool area exposed to the shielding gas. The situation then approached the equilibrium conditions that were expected from the gas nitrogen activity and weld pool alloy composition according to thermodynamic calculations using the Thermo-Calc database.

Influence of Shielding Gas Composition and Welding Parameters on the N-content and Corrosion Properties of Welds in N-alloyed Stainless Steel Grades

A materials data base has been set up with mechanical, thermophysical and transformation properties of tool and creep resistant steels. In particular powder metallurgical materials have been covered. The intended use of the data is for numerical simulation of heat treatment with focus on computation of residual stresses and distortion in compound materials (macro composites). The main body of the data was generated in new experiments. In this paper data for the powder metallurgical high speed steel ASP 2060 will be presented. The material properties determined include thermophysical data such as thermal conductivity and specific heat, mechanical data such as yield strength and work hardening rate, and thermal expansion data. Most data were determined at different temperatures and after heat treatments to different microstructures. Additional data added to the database include literature data for elastic properties and transformation induced plasticity. Quenching of a compound ring consisting of an inner high speed steel and an outer tool steel was simulated numerically using two codes, DistSIMR and Trast7. With the help of a database interface input data adjusted to the formats of the two codes were generated. The results of the simulations showed good agreement between the two models both for the computed residual stresses and distortions, which confirms the reliability of the results.

Roland Blom

Papers

Influence of Shielding Gas Composition and Welding Parameters on the N-content and Corrosion Properties of Welds in N-alloyed Stainless Steel Grades

Material property database for heat treatment simulation of tool materials