Press Hardening Coating Strategy Against Hydrogen Delayed Cracking
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Solution Overview
Problem
The sensitivity to delayed cracking increases with the mechanical strength of steel sheets, particularly after cold-forming or hot-forming operations, due to high residual stresses and hydrogen absorption, which can lead to intergranular cracking.
Innovation Solution
A press hardening method involving a steel sheet precoated with a zinc- or aluminum-based pre-coating and a hydrogen barrier pre-coating, followed by batch annealing in an inert atmosphere to reduce hydrogen absorption and promote a microstructure resistant to delayed cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the mechanical strength of steel sheet is increased, then the resistance to impact and structural integrity is improved, but the sensitivity to delayed cracking increases due to high residual stresses and hydrogen absorption
Solution Approach 1:
A hydrogen barrier pre-coating is applied to the steel sheet before hot-forming operations. This pre-coating acts as a protective barrier that prevents hydrogen ingress during the heating and forming processes, thereby preventing delayed cracking before it can occur
Solution Approach 2:
The steel sheet is processed in a controlled atmosphere environment during batch annealing and hot-forming. This inert atmosphere prevents external hydrogen sources from contaminating the steel, thereby reducing hydrogen absorption while maintaining high mechanical strength
2Stability of the object's composition
If batch annealing is performed for extended periods to achieve desired microstructure, then the mechanical properties and microstructure are improved, but hydrogen absorption increases significantly over time
Solution Approach 1:
The hydrogen barrier pre-coating is applied before the batch annealing process. This pre-coating remains intact during the extended annealing period, continuously protecting the steel sheet from hydrogen absorption while the desired microstructure develops over time
Solution Approach 2:
The hydrogen barrier pre-coating serves multiple functions simultaneously: it protects against hydrogen absorption during extended batch annealing, maintains corrosion resistance, and allows the steel to achieve the desired microstructure without compromising any of these properties
3Weight of moving object
If high strength steel is used for automotive parts, then the weight reduction and impact resistance are achieved, but the residual stresses from deformation lead to delayed cracking
Solution Approach 1:
The hydrogen barrier pre-coating is applied to high strength steel sheets before forming operations. This pre-coating prevents hydrogen ingress during the hot-forming process, allowing the steel to be formed into lightweight automotive parts without developing delayed cracking
Solution Approach 2:
The steel sheet is processed through controlled heating and cooling cycles during hot-forming. These parameter changes allow the steel to be formed at elevated temperatures where it is more ductile, then cooled to achieve the desired high strength properties without excessive residual stresses that would cause cracking
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
The method significantly reduces hydrogen absorption into the steel sheet and enhances the resistance to delayed cracking, achieving a microstructure with improved mechanical properties and corrosion resistance.
Implementation Method 1
hydrogen absorption into the pre-alloyed aluminum-based steel sheet and therefore into the press hardened part is prevented
Implementation Method 2
Hydrogen may progressively build up by diffusion into the crystal lattice defects
Implementation Method 3
the thermal treatment of the blank to obtain a fully austenitic microstructure in the steel
Implementation Method 4
the cooling of the part obtained at step G) in order to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75% in terms of volume fraction of equiaxed ferrite
Data Source
AI summary
A press hardening method includes the following steps: A. the provision of a steel sheet for heat treatment, precoated with a zinc- or aluminum-based pre-coating for anti-corrosion purpose, B. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm, C. the batch annealing of the precoated steel sheet in an inert atmosphere to obtain a pre-alloyed steel sheet, D. the cutting of the pre-alloyed steel sheet to obtain blank, E. the thermal treatment of the blank to obtain a fully austenitic microstructure in the steel, F. the transfer of the blank into a press tool, G. the hot-forming of the blank to obtain a part, H. the cooling of the part obtained at step G).