Hot Working of Nb Austenitic Stainless Steel Bars Without Surface Cracks
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Solution Overview
Problem
Traditional hot working methods for niobium-containing high-alloy austenitic heat-resistant stainless steel bars result in surface cracking and low production efficiency due to the formation of massive niobium compounds and poor thermoplasticity, especially when producing smaller bars.
Innovation Solution
A hot working method involving heating, radial forging, and hot rolling with optimized temperature and time controls to break the as-cast structure, form a recrystallized structure, and prevent cracking, including specific temperature ranges and deformation rates for each process step.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hot working methods are used for niobium-containing high-alloy austenitic stainless steel, then the material can be processed, but surface cracking occurs and production efficiency is low due to poor thermoplasticity
Solution Approach 1:
The patent applies parameter changes by precisely controlling heating temperature (1200-1270°C), holding time (calculated by T1=0.5D+600×w(Nb)×100), and deformation parameters during radial forging and hot rolling. This resolves the contradiction by creating optimal processing conditions that improve thermoplasticity while preventing surface cracking, thereby enhancing both manufacturability and surface quality.
Solution Approach 2:
The patent applies preliminary action through a specific sequence: heating the steel ingot to 1200-1270°C, holding for calculated time to dissolve Nb compounds, then radial forging with total compression ratio >3 to break as-cast structure before hot rolling. This preliminary heating and structural preparation improves thermoplasticity in advance, preventing surface cracking during subsequent forming operations.
2Quantity of substance
If the size of raw material bars is reduced to produce smaller finished products, then production costs are reduced, but repeated firing forging is needed due to too many passes and large temperature drop, rendering production efficiency too low
Solution Approach 1:
The patent applies parameter changes by using high heating temperature (1200-1270°C) and calculating appropriate holding time (T1=0.5D+600×w(Nb)×100) based on ingot diameter and Nb content. This ensures sufficient heat penetration and Nb compound dissolution even for smaller ingots, allowing direct radial forging in one heat treatment and eliminating the need for repeated firing forging, thus maintaining high production efficiency while using smaller raw materials.
3Shape
If stainless steel ingot is subjected to conventional cogging by primary rolling+hot rolling or cogging by primary rolling+forging to produce small size bars, then the material can be formed, but surface cracking is serious
Solution Approach 1:
The patent applies inversion by reversing the conventional sequence: instead of primary rolling first then hot rolling or forging, it performs radial forging (cogging) first to break the as-cast structure and dissolve Nb compounds, then followed by hot rolling. This inverted sequence prevents surface cracking by preparing the material structure in advance, allowing conventional hot rolling to proceed without causing surface defects.
4Strength
If high contents of Cr and Ni are added to improve high temperature resistance, then creep resistance is improved, but massive niobium compounds form during solidification, significantly reducing thermoplasticity
Solution Approach 1:
The patent applies parameter changes by heating to high temperature (1200-1270°C) and holding for calculated time (T1=0.5D+600×w(Nb)×100) to completely dissolve massive Nb compounds formed during solidification. This resolves the contradiction by eliminating the harmful Nb compounds through controlled heating, thereby restoring thermoplasticity while preserving the high Cr and Ni content that provides creep resistance.
Solution Approach 2:
The patent applies blessing in disguise by utilizing the high heating temperature (1200-1270°C) and extended holding time not only to dissolve Nb compounds (removing harm) but also to create a uniform austenitic structure and promote grain refinement (creating benefit). The same thermal treatment that eliminates the thermoplasticity problem also enhances the material's overall structure, making the material both manufacturable and high-performance.
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 produces niobium-containing high-alloy austenitic stainless steel bars with good surface quality and uniform grain size, enhancing production efficiency and preventing surface cracks.
Implementation Method 1
heating a steel ingot at a heating temperature in a range from 1,200° C. to 1,270° C.
Implementation Method 2
subjecting the heated steel ingot to cogging by radial forging at a total compression ratio greater than 3
Implementation Method 3
break the as-cast structure, form a recrystallized structure
Implementation Method 4
heating the square billet at a heating temperature in a range from 1,200° C. to 1,270° C.
Implementation Method 5
subjecting the heated square billet to hot rolling to obtain a bar
Data Source
AI summary
The present disclosure relates to a hot working method for stainless steel bars, comprising: step (1) heating a steel ingot at a heating temperature in a range from 1200° C. to 1270° C.; step (2) subjecting the heated steel ingot to radial forging and cogging down according to a total compression ratio greater than 3 to obtain a square billet; step (3) heating the square billet at a heating temperature in a range from 1200° C. to 1270° C.; and step (4) subjecting the heated square billet to hot rolling to obtain a bar. The processing method of the present disclosure can avoid the quality defect of surface crack while ensuring production efficiency by optimizing the process flow and controlling key process parameters, so that niobium-containing high-alloy austenitic heat-resistant stainless steel bars with good surface quality and structure can be prepared.