T-TRIP: Investigation of transformation induced plasticity during precipitation formation in quenched and tempered steels and maraging steels

• Applicant: Professor Dr.-Ing. Volker Schulze
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428958028

• Final Report Year: 2025

Final Report Abstract

Surface hardening processes are designed to create a hard outer layer with a ductile core and induce compressive residual stresses to enhance fatigue properties. This hardened layer can be intentionally or unintentionally tempered through processes such as grinding or reheating, e.g., during laser line hardening. In additive manufacturing, this tempering occurs within the heat-affected zone due to thermal gradients and induced residual stresses. These processes lead to stress-influenced precipitation, where microstructural mechanisms like transformation plasticity (TRIP), which describes plastic deformation induced by phase transformation under stress below the material's yield strength, play a crucial role. The project aimed to investigate TRIP during precipitation to address existing research gaps. Two groups of steels were analyzed: three quenched and tempered steels and a bearing steel, where TRIP during tempering led to increased ductility, as well as three maraging steels examined in the context of TRIP in additive manufacturing and precipitation strengthening during aging. Parameters such as tempering temperature, heating rate, stress level, stress direction, and chemical composition were varied to understand their impact on TRIP. Microscopic analyses (SEM, TEM) and hardness measurements provided detailed insights into the type, size, and fraction of precipitates under different experimental conditions. The study revealed that the TRIP constant was closely linked to the precipitate area fraction, while carbon content alone was insufficient for accurately predicting TRIP, highlighting the need to consider multiple factors. The developed models and findings enable improved simulations of heat treatment processes, allowing for more precise predictions of material behavior and better control of material properties. These results open new avenues for the targeted adjustment of processes to enhance component properties.

Publications

• Characterization of a novel maraging steel for laser-based powder bed fusion: optimization of process parameters and post heat treatments. Journal of Materials Research and Technology, 18, 931-942.
  Nouri, Niki; Li, Qing; Damon, James; Mühl, Fabian; Graf, Gregor; Dietrich, Stefan & Schulze, Volker
  
• Characterization of phase transformation and strengthening mechanisms in a novel maraging steel produced using laser-based powder bed fusion. Materials Characterization, 207, 113522.
  Nouri, Niki; Li, Qing; Schneider, Reinhard; Damon, James; Schüßler, Philipp; Laube, Stephan; Müller, Erich; Graf, Gregor; Schulze, Volker & Dietrich, Stefan
  
• Transformation Induced Plasticity (Trip) Due to Precipitation During Continuous Tempering of Quenched and Tempered Steels. Elsevier BV.
  Nouri, Niki; Hillenmeyer, Elena; Thavarajan, Thanusan; Dietrich, Stefan & Schulze, Volker
  
• Transformation Induced Plasticity during Precipitation Strengthening of Maraging Steels. Heat Treating Conference, 84901, 50-56. ASM International.
  Nouri, Niki; Dietrich, Stefan & Schulze, Volker