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Author: Site Editor Publish Time: 2025-01-02 Origin: Site
Abstract: This article focuses on the field of heat treatment, conducting an in-depth exploration of common misunderstandings and practical operational issues in the heat treatment process across the industry. Topics covered include vacuum heating, tempering color judgment, mold failure reasons, and hardness control, among others. The aim is to clarify misconceptions, enhance scientific understanding of heat treatment processes, assist professionals in optimizing procedures, avoid disputes, and promote the healthy development of the industry.
During vacuum heat treatment, the phenomenon of carbon uptake by workpieces often leads to misconceptions. Some mistakenly attribute the cause to two factors: one, carbon uptake from quenching oil; and two, graphite components in the heating chamber. However, the real issue is often the cleanliness of the heating chamber. When workpieces are moved in and out of the furnace, along with the material baskets and loading carts, quenching oil contaminates the chamber, which forms a volatile reducing atmosphere that contributes to carbon uptake.
It’s important to note that if workpieces are oil-quenched at temperatures below 1050°C, and if they are pre-cooled before entering the oil, carbon uptake is typically not significant. While graphite components in the heating chamber do contribute to carbon uptake, their impact is far less significant than the contamination caused by residual quenching oil.
Distortion during heat treatment involves both microstructural changes and shape distortions. Studies have shown that, under identical organizational and hardness conditions, vacuum heat treatment results in minimal microstructural distortion compared to other furnace types. However, in terms of shape distortion, vacuum heat treatment does not always have the upper hand. Other processes, such as conventional quenching, can easily control distortion using methods like stepwise quenching, isothermal quenching, and furnace-side straightening. In contrast, vacuum quenching may sometimes exacerbate distortion due to the lack of such functional capabilities.
A common misunderstanding is that vacuum heat treatment minimizes all forms of distortion, which could lead to improper process selection in practice.
The color formed on the surface of steel after tempering, due to the oxide film, is often used to estimate tempering temperature. Tempering color does indeed change with temperature, with standard times around 5 minutes at different temperatures resulting in characteristic colors for various steels (e.g., carbon steel at 200°C is light yellow, at 220°C it is grass yellow, etc.).
However, it’s crucial to emphasize that tempering color is influenced by both temperature and time. At the same temperature, extending the holding time will result in a color that corresponds to a higher temperature. Judging tempering temperature solely by color can lead to inaccurate conclusions without considering the time factor.
Statistical data on early mold failure, both domestic and international, shows that improper heat treatment processes account for a significant proportion of failures (44% in Japan, 52% in Shanghai). However, this merely reflects past failures and cannot be used to predict future mold failure causes.
Many people fall into the trap of attributing mold failures to heat treatment alone, but mold failure is a complex process that requires comprehensive analysis of material quality, design, processing methods, material properties, and other factors.
The core goal of forging is to eliminate material defects, improve microstructure, enhance material properties, and reduce machining costs. However, some in the forging industry focus solely on ensuring dimensional accuracy, neglecting material optimization, and using improper post-forging heat treatments that result in harmful carbide networks.
Since forging temperatures are often much higher than quenching temperatures, this poor microstructure can adversely affect heat treatment and product quality. This underscores the importance of maintaining high forging quality to ensure effective heat treatment results.
Heat treatment is not just about meeting the hardness requirement. Process selection and control are also critical. Some practitioners may use underheating during quenching, and then adjust tempering to barely meet the specified hardness range, but this increases the risk of early failure, as such processes fail to ensure the proper transformation of internal microstructures.
Hardness compliance cannot mask the flaws in heat treatment processes, and improper heat treatment can lead to brittleness and cracking issues during product use.
In quality disputes, customers often assume that a product is flawless before heat treatment and blame heat treatment if issues arise, demanding compensation. However, this view ignores the fact that heat treatment is just one part of the manufacturing chain, and its quality depends on previous steps. Proper pre-treatment conditions must be created for heat treatment to be effective.
Some people rely on handbook hardness values for design, only to find they cannot achieve the required hardness during heat treatment. This is common when, for example, using large components made from spring steel (e.g., 60Si2Mn), where achieving hardness values from the handbook is difficult due to the varying thickness of the workpiece.
Factors such as material grade, mold size, workpiece weight, shape, and subsequent processing methods all affect the final hardness outcome, requiring adjustments to meet practical conditions and ensure proper material performance.
In cases of product failure, such as a mold fracture that causes injury, some companies quickly hold the heat treatment manufacturer responsible. However, this overlooks the fact that product failure is a multifactorial issue, involving design, material selection, processing, and usage.
As in medical diagnosis, a thorough analysis of each stage is necessary to identify the true cause of failure. For example, in one case, the heat treatment was deemed normal, and the root cause of failure was found to be overloading during use. A scientific and rigorous approach to problem-solving is essential.
Some customers impose strict hardness requirements on heat-treated products, demanding deviations of no more than 1 HRC, even though hardness testing equipment allows for such a variation. This often leads heat treatment manufacturers to use unreasonable methods to meet these demands, creating operational difficulties.
This excessive demand reflects a lack of understanding of the heat treatment process and harms the industry’s healthy development. Both parties should work together, based on scientific understanding, to establish reasonable tolerances.
It is commonly believed that tempering should only occur after cooling to room temperature, but this is incorrect for many steels, particularly low- and medium-carbon steels. In these cases, it’s better to proceed with tempering while the workpiece is still above room temperature, to reduce the risk of cracking and improve product quality.
Some practitioners mistakenly believe that workpieces should always be tempered while still hot from quenching. In fact, the furnace entry temperature for tempering should be based on the steel's martensite transformation point, which varies by material. Blindly following this practice can lead to cracking and diminished material properties.
Some managers insist on letting molds sit at room temperature for a week between annealing and quenching to release stress, but this claim lacks scientific support. Extending production cycles unnecessarily, based on unsupported practices, should be avoided. Heat treatment processes should follow a scientifically justified schedule.
Some customers attempt to save on processing costs by completing all dimensional machining before heat treatment and then expect the heat treatment process to cause no distortion. Since heat treatment inherently causes microstructural distortion, some dimensional changes are inevitable.
This practice, which shifts cost-saving pressures onto heat treatment, is unreasonable. Companies should consider both machining costs and product quality, leaving room for necessary dimensional adjustments after heat treatment.
Some companies neglect to remove decarburized layers before testing hardness, leading to erroneous hardness readings. Correct procedures should involve removing the decarburized layer before testing to ensure accurate results.
The iron-carbon phase diagram is essential for understanding heat treatment processes, but it only applies to simple alloys like carbon steels and cast irons. For alloy steels, it’s less applicable due to the presence of other elements. Furthermore, the rapid heating and cooling during heat treatment do not reach equilibrium conditions, so the phase diagram is more of a starting point than a practical guide for process operations.
In low-carbon steel annealing, it's often assumed that equiaxed grains can be easily obtained. However, in certain steels like Al-stabilized steels, grain formation may be affected by prior processing and result in a more deformed structure, even under high-temperature annealing.
In extrusion, it’s believed that lower hardness facilitates material deformation, but in fact, pearlitic spheroidized structures, though harder, deform better. Material pre-treatment should focus on optimizing both hardness and deformation properties.
Forging mold users often demand excessively high hardness (e.g., 52-55 HRC), but this can reduce red hardness, causing rapid hardness loss. Instead, maintaining proper red hardness within a reasonable hardness range results in better mold performance.
The idea that “perfect heat treatment results in perfect parts” is misleading. Heat treatment is just
one link in the chain of production, and a holistic approach to quality is needed. Other factors like material defects, design, and machining play critical roles.
Conclusion:In heat treatment, precision in both knowledge and practice is paramount. Misunderstandings can lead to ineffective processes and product failures. By addressing common misconceptions and applying a scientific, holistic approach to heat treatment, the industry can ensure higher quality, reduced disputes, and a healthier development trajectory. Clear communication and a deep understanding of the entire production chain are essential for overcoming the complex challenges of heat treatment.
