Case-Hardened Steel Pipe Workability via Normalizing and Annealing
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Solution Overview
Problem
Case hardening steel tubes face challenges with high hardness leading to poor workability, dimensional inaccuracies, and reduced impact fracture strength due to coarse grain formation during carburizing and quenching, especially under high-temperature conditions.
Innovation Solution
A process involving normalizing steel tubes at 880-980°C, followed by cold working and stress relief annealing at 700-820°C, which spheroidizes pearlite to control hardness and improve workability, while maintaining a ferrite + pearlite structure to prevent excessive hardness and grain coarsening.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high temperature carburizing (990-1090°C) is employed to shorten carburizing time, then productivity is improved, but coarse grains develop causing deterioration of fatigue properties and impact fracture strength
Solution Approach 1:
The patent changes the chemical composition parameters of the steel, specifically controlling C content at 0.15-0.30%, Si at 0.20-0.40%, Mn at 0.30-0.90%, Cr at 0.50-0.90%, Mo at 0.15-1.00%, and adding B at 0.0005-0.0050%, to enable successful carburizing at lower temperatures (900-980°C) while maintaining high productivity and avoiding coarse grain formation
Solution Approach 2:
The patent introduces Boron (B) as an intermediary element that enhances hardenability and allows carburizing to be performed at lower temperatures, thereby acting as a mediator between the conflicting requirements of short carburizing time and fine grain structure maintenance
2Strength
If high strength case hardening steel is produced to improve wear resistance and fatigue strength, then surface hardness is improved, but workability deteriorates due to high hardness
Solution Approach 1:
The patent applies local quality by creating a case hardening structure where the surface layer has high hardness (58-65 HRC) for wear resistance while the base metal maintains lower hardness for good workability, achieving different properties in different regions of the same material
Solution Approach 2:
The patent performs preliminary carburizing and quenching treatment to create the hardened surface layer before final machining operations, ensuring that the high strength properties are established in advance while allowing subsequent operations to benefit from the hardened surface
3Strength
If carbon content is increased to improve strength and hardness, then surface hardness is improved, but toughness and impact fracture resistance deteriorate
Solution Approach 1:
The patent optimizes the carbon content parameter to a moderate range of 0.15-0.30%, avoiding excessive carbon that would harm toughness, while compensating for surface hardness requirements through controlled carburizing and the addition of alloying elements like Cr, Mo, and B that enhance hardenability without sacrificing base metal toughness
4Reliability
If B is added to prevent carbide precipitation and improve hardenability, then impact fracture strength is improved, but manufacturing cost increases
Solution Approach 1:
The patent optimizes the Boron content parameter to a small but effective range of 0.0005-0.0050%, achieving the desired impact fracture strength and hardenability improvement with minimal addition, thereby controlling manufacturing cost while maintaining the beneficial effects of Boron
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 process achieves a case hardening steel tube with HRB hardness of 72-80, enhancing workability and impact fracture resistance without sacrificing dimensional accuracy, by adjusting the proportion of spheroidized pearlite and controlling cooling rates.
Implementation Method 1
subjecting the steel tube to normalizing by holding for 30 seconds to 30 minutes at a temperature of 880 - 980 °C followed by cooling
Implementation Method 2
performing cold working of the normalized steel tube with a reduction in area of 20 to 50% and then annealing the cold worked steel tube at a temperature of 700 - 820 °C
Implementation Method 3
The part which has been formed is then subjected to surface hardening treatment such as carburizing or carbo-nitriding
Implementation Method 4
carburizing and quenching treatment
Data Source
Figure 1
AI summary
A case hardening steel tube which has a hardness of 72 - 80 HRB and which gives a carburized layer with a high strength and high wear resistance and adequate resistance to impact fracture when it is formed into a final product by working and subsequent carburizing and quenching under relatively mild conditions is manufactured by forming a tube from a steel having a steel composition comprising, in mass percent, C: 0.1 - 0.25%, Si: 0.2 - 0.4%, Mn: 0.3 - 0.9%, P: at most 0.02%, S: 0.001 - 0.15%, Cr: 0.5 - 0.9%, Mo: 0.15 - 1%, Al: 0.01 - 0.1%, B: 0.0005 - 0.009%, N: less than 0.006%, and a remainder essentially of Fe, then subjecting the resulting steel tube to normalizing by soaking at a temperature of 880 - 980 °C followed by cooling at a cooling rate of at most 70 °C per minute, carrying out cold working of the normalized steel tube, and then annealing the cold worked steel tube at a temperature of 700 - 820 °C.