Link Chain Low-Temperature Toughness via Two-Stage Quenching
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Solution Overview
Problem
Conventional link chains face challenges in achieving a balance between strength and toughness, particularly at low temperatures, as they fail to meet the demands of high tensile strength and low temperature toughness required in modern usage environments.
Innovation Solution
A two-stage quenching-tempering heat treatment method is applied to low carbon steel (MnB steel) wire to produce a link chain with a finer austenite grain size and a balanced structure of martensite and carbonitride precipitates, ensuring superior strength and toughness even at -40°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional single-stage quenching-tempering heat treatment is applied to achieve high strength, then tensile strength can reach 930N/mm², but low temperature toughness deteriorates with Charpy impact value insufficient for -40°C environments
Solution Approach 1:
The heat treatment process is divided into two distinct stages: first quenching at 830-950°C followed by tempering at 400-600°C, then a second quenching at 750-850°C followed by a second tempering at 150-350°C. This segmentation allows independent optimization of strength (achieved in first stage) and low-temperature toughness (achieved in second stage), resolving the contradiction between high strength and low temperature reliability.
Solution Approach 2:
The invention changes multiple parameters systematically: heating temperatures (830-950°C then 750-850°C), cooling rates (rapid quenching), and tempering temperatures (400-600°C then 150-350°C). These parameter changes transform the microstructure from conventional coarse-grained martensite to fine-grained martensite with controlled precipitates, achieving both high strength (1000-1500N/mm²) and excellent low-temperature toughness (Charpy impact value 45J or more at -40°C).
2Reliability
If high frequency induction heating is used to obtain mixed martensite-ferrite structure with fine grain size, then both strength and toughness can be achieved, but strict temperature control is required making the process impractical
Solution Approach 1:
The invention segments the heat treatment into two distinct stages with different temperature ranges and holding times. The first stage (830-950°C) creates austenite grains, while the second stage (750-850°C) refines them. This segmentation eliminates the need for strict single-point temperature control in the austenite+ferrite coexisting region, simplifying the process while achieving fine-grained martensite structure with both strength and toughness.
Solution Approach 2:
The invention changes the heating temperature to a higher range (830-950°C) for the first quenching, which is above the conventional single-stage temperature, allowing complete austenite formation without ferrite. Then the second quenching uses a lower temperature (750-850°C) to refine grains. This parameter change strategy achieves fine grain structure without requiring strict control in the problematic coexisting region.
3Strength
If conventional heat treatment produces coarse grain size of 6.5, then high strength can be achieved, but low temperature toughness is insufficient for modern usage environments
Solution Approach 1:
The two-stage quenching process segments the grain formation and refinement into separate steps. The first quenching (830-950°C) establishes the base austenite grain structure, while the second quenching (750-850°C) refines these grains to a fine size of 9-12. This segmentation allows achieving both high strength (1000-1500N/mm² breakage stress) and fine grain structure for low-temperature toughness.
Solution Approach 2:
The invention systematically changes the heating temperature parameters: first stage at 830-950°C to form austenite grains, then second stage at 750-850°C to refine them. This parameter change strategy transforms the conventional coarse grain size (6.5) to fine grain size (9-12), achieving both high breakage stress (1000-1500N/mm²) and excellent low-temperature impact resistance (45J or more at -40°C).
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 results in a link chain with a Charpy impact value of 45J or more at -40°C, significantly improving low temperature toughness and maintaining strength, outperforming conventional single-stage treatments by achieving twice the impact value.
Implementation Method 1
treating a link chain produced from low carbon steel (MnB steel) wire by a two-stage quenching-tempering treatment
Implementation Method 2
two-stage quenching-tempering treatment so as to make the link chain structure a martensite single phase
Implementation Method 3
make the austenite grain size a finer 9 to 12
Implementation Method 4
strict control of the precipitates contained in the link chain
Data Source
Figure 1(a)~1(b)
Figure 2
AI summary
The present invention provides a link chain superior in low temperature toughness comprised of low carbon steel, said link chain characterized in that two-stage high frequency quenching-tempering is performed to give a link chain which has a martensite single phase structure, has an austenite grain size of 9 to 12, further contains in said structure a precipitate comprised of 40 nm or smaller TiC in 90% or more and 80 nm or smaller Ti(C,N) in 10% or less, contains AlN in not more than 0.0003%, and has a -40°C Charpy impact value of at least 45J.