Carburized Steel Grain Control via Nitride Pinning
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional carburizing techniques fail to provide mechanical parts with satisfactory fatigue properties against both low and high load inputs, as they either result in inadequate surface hardness or inner hardness, leading to compromised workability and crystal grain coarsening due to denitrification during the carburizing process.
Innovation Solution
A carburized part with a specific steel composition and manufacturing method, where the surface layer has a crystal grain size greater than 5 and the inner portion has a grain size of 5 or less, achieved by controlling the amount of nitride particles through a vacuum carburizing treatment with a nitriding gas, maintaining the required nitride particle concentration using Equation (1) to prevent coarsening and denitrification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a carburizing treatment is performed at high temperature to improve surface hardness, then surface strength is improved, but crystal grains in the surface layer become coarse
Solution Approach 1:
Nitride particles are precipitated in advance during the steel manufacturing step before carburizing treatment. These pre-formed nitride particles serve as pinning particles that will prevent crystal grain coarsening during the subsequent high-temperature carburizing process
Solution Approach 2:
The invention changes the chemical composition parameters of the steel by adding specific amounts of Al (0.02-0.10%), Nb (0.01-0.20%), and Ti (0.005-0.20%) along with controlled nitrogen content (0.025% or less). These compositional changes enable the formation of sufficient nitride particles that will pin crystal grain boundaries during high-temperature treatment
2Stability of the object's composition
If large amounts of N and Al or Nb are added to precipitate nitride particles as pinning particles, then crystal grains are prevented from becoming coarse, but the steel composition becomes complex and costly
Solution Approach 1:
The invention optimizes the parameter ranges of alloying elements to achieve effective nitride particle formation without excessive additions. By specifying precise ranges for Al (0.02-0.10%), Nb (0.01-0.20%), Ti (0.005-0.20%), and N (0.025% or less), the invention achieves crystal grain stabilization with controlled, economical composition
Solution Approach 2:
The invention creates a composite microstructure consisting of the steel matrix combined with dispersed nitride particles (AlN, NbN, TiN). These nitride particles function as reinforcement phases that pin crystal grain boundaries, providing a composite material solution that achieves grain refinement without complex overall composition
3Object-affected harmful factors
If nitriding gas is introduced during vacuum carburizing to prevent denitrification, then nitrogen is supplied to the surface layer, but the relationship between nitride particle quantity and crystal grain size is not controlled
Solution Approach 1:
Instead of relying on nitriding gas introduction during carburizing, the invention performs the critical action of precipitating nitride particles in advance during steel manufacturing. This preliminary formation of pinning particles ensures precise control over their quantity and distribution before the carburizing process begins
Solution Approach 2:
The invention replaces the process control approach (introducing nitriding gas during carburizing) with a material composition approach (controlling alloying element content to precipitate sufficient nitride particles). This substitution transfers the control mechanism from process parameters to material parameters, achieving more precise and predictable results
Data Source
AI summary
The present invention provides a carburized part which is formed by processing a steel into a shape of a part and performing a carburizing treatment on the steel, the steel having a composition consisting essentially of, in terms of % by mass: 0.10% to 0.40% of C; 0.05% to 2.00% of Si; 0.30% to 2.00% of Mn; 0.30% to 3.00% of Cr; 0.025% or less of N; and as a pinning particle forming element which forms a pinning particle by nitrification, one or two or more elements selected from: 0.020% to 0.100% of Al; 0.01% to 0.20% of Nb; and 0.005% to 0.20% of Ti, and optionally: 0.80% or less of Mo, with the remainder being Fe and inevitable impurities, in which a crystal grain size number of a surface layer of the part at a depth of 50 μm or less from a surface is greater than 5, and the crystal grain size number of an inner portion of the part at a depth of 3 mm or more from the surface is 5 or less.