Simulation Platform for the Integrative Development of Materials and Process Chains

Development of Aluminum-Free Case Hardening Steel

Aluminum-Free Case Hardening Steel IEHK RWTH Aachen University

In this project, a simulation platform for the integrative development of materials and process chains is introduced. This simulation platform is applied to the numerical development of aluminum-free case hardening steel with improved cleanliness for large gears in the wind industry. In order to validate the new numerical based combined material and process concept, new gear steel developed by simulation will be produced on a laboratory scale, forged to a bevel gear and carburized to demonstrate its suitability for high-temperature carburization. In future, the industrial implementation will be established with a consortium of industrial partners along the entire process chain for large gear components.

Practical Issues

Aluminum-Free Case Hardening Steel

Improving the service life of carburized gear components is a high priority for wind turbine manufacturers. Only a reliable prevention of early failures ensures economic operation of wind turbines. The service life of gear components depends on many factors which are set during material production, forming or heat treatment. Important influencing parameters such as phase fraction, grain size and grain size distribution can be reset again by an adapted heat treatment at any point in the process chain. However, microscopic oxide purity is an important endurance limiting issue, which is set during the metallurgical production of steel and is irreversible from then on in the solid material. The improvement of the level of purity without deterioration of the fine grain stability is a high priority for steel manufacturers and wind turbine operators.

Approach

Integrative simulation of precipitation IEHK RWTH Aachen

A material based approach for improving the oxide purity level represents the reduction of the aluminum content, which decreases the likelihood of lifetime-reducing aluminum oxides in the material and thus a premature failure of the wind turbine. However, aluminum is used in the further course of the process chain in combination with a defined addition of nitrogen also for ensuring the fine-grain resistance, which in turn is responsible for setting the tooth strength. Metallurgically this effect is based on aluminum nitrides that are finely dispersed in the microstructure which exist as finely dispersed precipitates on the nanometer scale and reduce the grain boundary mobility.

Technical Challenges

Purity grade-related early failures are to be avoided for the economic use of wind turbines. Aluminum-free microalloyed case hardening steels show an improved microscopic oxide purity level and promise a reliable use of large gears. Moreover, they are produced time and energy reduced by the so-called high-temperature carburising process allowing an efficient manufacturing route. The development of the current aluminum-free alloy concept is based on a combination of modern numerical simulation methods and aims at the substitution of aluminum nitrides by niobium carbonitrides. The basis for the combination of different model approaches is a simulation platform for the calculation of process chains, which allows the calculation at all relevant levels of observation at the nanoscale of the precipitates, at the micro scale of the microstructure and at the macro scale of the components. Based on this multiscale approach the optimisation of alloys and process parameters can be realised in a holistic manner. In order to implement an integrative, simulation-based development of materials and processes, several different simulation approaches and programmes are necessary. The efficient and effective use of various programmes is ensured by developing an Internet-based platform for the simulation of process chains. In doing so, uniform formats for data and visualisation were defined. This approach allows simultaneous simulation and analysis on all relevant scales from nano- to component-scale. The test case “Al-free case hardening steel” deals with quantification of the fraction and size evolution of microalloying precipitates along different alternative process chains. On this basis it allows to predict the finegrain stability during carburisation as a function of the process chain, the process parameters and the chemical composition. For validation of the purely numerically developed alloy concept, a laboratory melt was created, processed into a bar and examined with respect to purity and fine grain resistance at high carburising temperatures. Eventually, good fine-grain stability has been documented for the modified variant after blank-hardening at 1050 C for 12 hours or at 1100 C for 1 hour for a process chain with FP-annealing.