Crankshaft Journal Hardening With Ferrite-Martensite Microstructure
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Solution Overview
Problem
Traditional hardening methods for medium carbon steel crankshafts result in excessive residual stress, leading to fatigue crack failures in internal combustion engine components.
Innovation Solution
A method involving heating the crankshaft journals to an intercritical temperature between 724° C. and 822° C., followed by quenching with a cooling rate of 15 to 20° C./sec, to achieve a microstructure comprising up to 50% ferrite and martensite, reducing residual stress while maintaining surface hardness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional hardening methods heat the journal surfaces to achieve 100% martensite microstructure, then surface hardness is improved, but residual stress becomes excessive leading to fatigue crack failures
Solution Approach 1:
The patent changes the heating temperature parameter from above Ac3 (to achieve 100% austenite) to the intercritical range between Ac1 and Ac3 temperatures. This parameter change results in a dual-phase microstructure of ferrite and martensite after quenching, which provides both adequate surface hardness and reduced residual stress, thereby improving fatigue resistance while maintaining strength requirements.
Solution Approach 2:
The patent creates a composite microstructure within the journal surface consisting of ferrite and martensite phases. This composite microstructure combines the advantages of both phases: martensite provides hardness and strength, while ferrite reduces residual stress and improves toughness, thus resolving the contradiction between surface hardness and fatigue resistance.
2Strength
If heating temperature is increased above Ac3 to achieve 100% austenite, then surface hardness is improved, but distortion and residual stress increase
Solution Approach 1:
The patent reduces the heating temperature parameter from above Ac3 to the intercritical range between Ac1 and Ac3. This temperature reduction decreases the thermal gradient and phase transformation stress during quenching, thereby reducing distortion while still achieving adequate surface hardness through the ferrite-martensite microstructure.
3Strength
If 100% martensite microstructure is achieved through quenching, then surface hardness is maximized, but subsurface residual stress becomes excessive
Solution Approach 1:
The patent changes the starting microstructure parameter before quenching from 100% austenite to a mixture of ferrite and austenite by heating to intercritical temperatures. During quenching, this results in a ferrite-martensite microstructure where the ferrite phase reduces subsurface residual stress while the martensite maintains surface hardness.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces residual stress by 15% to 30%, enhances surface hardness, and increases toughness, thereby preventing fatigue failures in crankshaft components.
Implementation Method 1
heating the surface of the first journal to an intercritical temperature
Implementation Method 2
quenching the surface of the first journal with a quench medium achieving a cooling rate between 15 to 20° C./sec
Implementation Method 3
the method further comprises tempering the workpiece
Data Source
AI summary
A method for manufacturing a crankshaft for an internal combustion engine with a plurality of journals having a hardened case with a first microstructure. The crankshaft is comprised of a steel comprising between about 0.3 wt % and 0.77 wt % Carbon. The first microstructure of the hardened case of the journals comprises between about 15% and 30% ferrite and a balance of martensite and the resultant subsurface residual stress between 310 MPa and 620 MPa.