Galvanized Steel Pipe Forming With Delayed Martensitic Hardening
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Solution Overview
Problem
Internal high-pressure forming methods have been unable to produce crack-free hardened galvanized steel pipes, as they often result in microcracking, making it difficult to use these methods for piping components.
Innovation Solution
A method involving pre-bending and pre-quenching of pipes, followed by austenitization and controlled cooling between 400 and 650°C, with active or passive cooling to achieve martensitic hardening, using a temperature-controlled medium for internal high-pressure forming, and optionally transferring the pipe to a cold tool for quench hardening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If internal high-pressure forming is used to form steel pipes, then complex geometries can be achieved, but microcracking occurs during hardening
Solution Approach 1:
The hardening process is segmented into two distinct phases: first cooling to 400-650°C to achieve partial hardening while the pipe remains flexible and formable, then completing the hardening after forming. This segmentation prevents microcracking by avoiding premature complete hardening during the forming operation.
Solution Approach 2:
The pipe is pre-cooled to 400-650°C before the actual forming operation. This preliminary cooling creates an intermediate state where the material has reduced ductility requirements but hasn't yet undergone complete martensitic transformation, allowing complex geometries to be formed without microcrack formation.
2Strength
If rapid cooling is applied to achieve martensitic hardening, then tensile strength increases to over 1500 MPa, but microcracking occurs
Solution Approach 1:
The cooling and hardening process is divided into two stages: first cooling to 400-650°C to achieve partial hardening while maintaining formability, then completing the rapid cooling to achieve full martensitic transformation. This segmentation allows strength development without microcrack formation.
Solution Approach 2:
The pipe undergoes preliminary cooling to 400-650°C before the final rapid cooling step. This preliminary action prepares the material structure to accept the subsequent rapid cooling without microcracking, while still achieving the desired martensitic hardness.
3Strength
If the pipe is cooled below 400°C before forming, then martensitic hardening begins, but the pipe becomes too brittle for forming operations
Solution Approach 1:
The pipe is preliminarily cooled to the specific temperature range of 400-650°C, which is sufficient to begin hardening but not so low as to cause excessive brittleness. This controlled preliminary cooling maintains the optimal balance between hardness development and formability.
Solution Approach 2:
The cooling temperature is precisely controlled within the 400-650°C range, changing the thermal parameter to achieve partial hardening while maintaining adequate ductility for forming. This parameter control prevents the material from becoming too brittle during the forming operation.
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 method reliably produces microcrack-free hardened steel pipes by ensuring the pipe is fully formed and hardened without premature hardening, allowing for complete removal from the mold and achieving the desired martensitic structure.
Implementation Method 1
the steel material must be heated to the austenitization temperature (>Ac3) and it is necessary to wait until the steel material is austenitized
Implementation Method 2
If after the austenitization, such a steel material is cooled at a speed that is above the critical hardening speed, then the austenitic structure converts into a martensitic, very hard structure
Implementation Method 3
the austenitic structure converts into a martensitic, very hard structure
Implementation Method 4
the heat flows out of the component and into the press tool, specifically at the speed greater than the critical hardening speed
Data Source
AI summary
The invention relates to a method for internal high-pressure forming and hardening of galvanized pipes made of sheet steel in which a pre-fabricated pipe is used; the pipe has at least one inlet opening (5) and a cavity (4); the pipe is heated to a temperature above the austenitization temperature (AC3) of the respective steel alloy and after the achievement of a desired degree of austenitization, is inserted into an internal high-pressure forming tool and acted on with a pressurized medium, which is forced into the cavity (4) through the at least one inlet opening (5) until the pipe fills a predetermined mold (2) of the tool, characterized in that the forming tool is heated to a temperature between 400 and 650° C., in particular 450-550° C., and the pressurized medium is likewise heated and has a temperature of 400-650° C.; after the austenitization, the pipe is allowed to passively cool or is actively cooled to a temperature of 400-600° C., but a temperature above the martensite starting temperature (Ms) of the selected steel alloy, and the cooling of the pipe for hardening purposes only takes place after the removal from the mold.