Ni-Cr Barrier Coating in Press Hardening Against Delayed Cracking
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Solution Overview
Problem
Carbon steel sheets used in press hardening methods for automotive applications face increased sensitivity to delayed cracking due to residual stresses and hydrogen absorption, which existing barrier pre-coatings with nickel and chromium do not adequately address, leading to insufficient resistance to delayed cracking.
Innovation Solution
A press hardening method involving a carbon steel sheet coated with a barrier pre-coating of nickel and chromium, with a weight ratio between 1.5 and 9, subjected to a thermal treatment in an atmosphere with oxidizing power between 1% and 50% oxygen volume, and a dew point between -30 and 30°C, followed by hot-forming to achieve a microstructure with enhanced resistance to delayed cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a barrier pre-coating comprising nickel and chromium with weight ratio Ni/Cr between 1.5 and 9 is applied to carbon steel sheet, then hydrogen absorption during austenitization treatment is reduced, but the resistance to delayed cracking remains insufficient
Solution Approach 1:
The patent applies parameter changes by precisely controlling the Ni/Cr weight ratio within 1.5-9, the thickness of the barrier pre-coating layer, and the austenitization temperature range (850-950°C) to optimize hydrogen barrier performance while maintaining mechanical properties and delayed cracking resistance
Solution Approach 2:
The patent uses composite materials by combining nickel and chromium in specific proportions to create a barrier pre-coating with enhanced hydrogen barrier properties, and further composite the steel matrix with controlled microstructure (martensite, bainite, and equiaxed ferrite phases) to achieve both strength and delayed cracking resistance
2Strength
If the mechanical strength of steel is increased to improve structural performance, then the sensitivity to delayed cracking increases due to residual stresses and hydrogen diffusion
Solution Approach 1:
The patent applies local quality by creating different microstructural phases (martensite for strength, equiaxed ferrite for ductility and hydrogen trapping) in specific proportions within the steel matrix, and applying a localized barrier pre-coating on the steel surface to provide hydrogen protection where needed
Solution Approach 2:
The patent uses the barrier pre-coating as an intermediary layer between the external environment (hydrogen source) and the steel substrate, preventing hydrogen from reaching the high-strength steel matrix during austenitization treatment
3Object-affected harmful factors
If thermal treatment is performed in inert atmosphere or nitrogen to prevent oxidation, then hydrogen absorption is reduced, but the barrier effect is not sufficient to prevent delayed cracking
Solution Approach 1:
The patent changes the atmospheric parameters by performing thermal treatment in air or oxidizing atmosphere with controlled oxygen content (5-50%) and dew point (-30 to +30°C), which creates a different mechanism for hydrogen barrier protection through oxide layer formation on the pre-coating surface
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 adsorption and enhances the barrier effect of the pre-coating, resulting in a part with excellent resistance to delayed cracking by forming thermodynamically stable oxides that inhibit hydrogen diffusion, thereby improving the mechanical properties of the steel.
Implementation Method 1
the thermal treatment of the blank in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen
Implementation Method 2
a barrier pre-coating which better inhibits hydrogen adsorption... forming thermodynamically stable oxides that inhibit hydrogen diffusion
Implementation Method 3
a barrier pre-coating which better inhibits hydrogen adsorption... significantly reduces hydrogen adsorption
Data Source
AI summary
A press hardening method includes the following steps providing a carbon steel sheet coated with a barrier pre-coating including nickel and chromium wherein the weight ratio Ni/Cr is between 1.5 and 9, cutting the sheet to obtain a blank, thermal treatment of the blank in an atmosphere having an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen, such atmosphere having a dew point between −30 and +30° C., transfer of the blank into a press tool, hot-forming of the blank to obtain a part, cooling of the part to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75 wt. % of equiaxed ferrite, from 5 to 20 wt. % of martensite and bainite in amount less than or equal to 10 wt. %.