Home
About
Equipment
Processing
Performance
Blog
HR
Contact
简体中文
86-13808367136
Company News
Industry News
Prospects for the Heat Treatment Industry
2024-04-30
What are the three stages of the heat treatment process?
2023-01-12
The three processes—heating, holding, and cooling—are sometimes reduced to just two: heating and cooling. These two processes are interconnected and cannot be interrupted.
Principles and Classification of Heat Treatment
Heat treatment is a process in which solid metals or alloys are heated, held at a specific temperature, and then cooled in a controlled manner to achieve the desired microstructure and properties.
What are overheating and underheating in heat treatment?
During heat treatment, overheating or underheating often occur. So, what exactly is overheating, and what is underheating?
Common Knowledge in the Heat Treatment Industry
Heat treatment furnaces, classified by process, include heating furnaces as well as furnaces for annealing, tempering, quenching, and more. Classified by furnace type, they include carriage-type heating furnaces, through-type furnaces, chamber-type forging furnaces, and others.
The “Annealing” Process Among the “Four Heat Treatments”
2019-05-21
Annealing is a heat treatment process in which a metal or alloy is heated to an appropriate temperature, held at that temperature for a specified period, and then slowly cooled—typically by allowing it to cool along with the furnace. The essence of annealing lies in heating steel to the austenitizing temperature and then inducing a pearlitic transformation. The microstructure obtained after annealing closely approximates the equilibrium microstructure. The primary objectives of annealing are as follows: (1) To reduce the hardness of steel and increase its ductility, thereby facilitating machining and cold deformation processes; (2) To homogenize the chemical composition and microstructure of the steel, refine the grain size, improve the steel’s mechanical properties, or prepare the microstructure for subsequent quenching; (3) To relieve internal stresses and work hardening, thus preventing deformation and cracking.
Q&A on Key Knowledge of Heat Treatment
Key Questions and Answers on Heat Treatment (Part 1) 1. Why do measuring tools need to undergo stabilization treatment? What is the typical stabilization treatment process for measuring tools? The treatment reduces the orthorhombic degree of martensite, transforming it into a more stable form of martensite and promoting the aging of retained austenite (A’). It also helps relieve residual stresses after quenching and deep cryogenic treatment, thereby providing excellent dimensional stability. 2. What are the two methods for ultrafine-grain processing of bearings, and what are their purposes? ① Purpose of pre-treatment by forging and quenching: This process can reduce the retained K content in A’ from 11.9% to 12.1% down to 7.11%, while achieving an A grain size of Grade 9 to 10. ② Purpose of double fine-grain processing for bearings: After this treatment, ...
Heating Defects and Their Control in Metal Heat Treatment
I. Overheating Phenomenon We know that overheating during heat treatment most easily leads to the coarsening of austenite grains, thereby reducing the mechanical properties of the part. 1. General Overheating Overheating occurs when the heating temperature is too high or the holding time at high temperature is excessively long, causing the austenite grains to become coarse. Coarse austenite grains can result in reduced strength and toughness of the steel, an elevated ductile-to-brittle transition temperature, and an increased tendency toward deformation and cracking during quenching. The causes of overheating typically include uncontrolled furnace temperature readings or improper material mixing—often due to a lack of process knowledge. Overheated microstructures can be refined by annealing, normalizing, or multiple high-temperature tempering treatments, which allow the austenite to re-austenitize under normal conditions, thereby refining the grain size.
Heat Treatment Process and Classification
Metal heat treatment is one of the crucial processes in mechanical manufacturing. Compared with other machining processes, heat treatment typically does not alter the shape or overall chemical composition of a workpiece; rather, it enhances or improves the workpiece’s service performance by modifying its internal microstructure or changing the chemical composition of its surface. A key characteristic of heat treatment is that it improves the intrinsic quality of the workpiece—quality that generally cannot be observed with the naked eye. As some people put it, machining is like surgery, while heat treatment is like internal medicine—a reflection of a nation’s core manufacturing competitiveness. The process itself generally comprises three stages: heating, holding (or soaking), and cooling. Sometimes, however, only the heating and cooling stages are involved.
Can you do tempering?
2020-04-03
Quenching and High-Temperature Tempering: This is a combined heat treatment process involving quenching followed by high-temperature tempering. Most parts subjected to quenching and tempering operate under relatively large dynamic loads, experiencing tensile, compressive, bending, torsional, or shear stresses. Some surfaces are also subject to friction, requiring a certain degree of wear resistance. In short, these parts work under various complex stress conditions. Such parts are primarily structural components in various machines and mechanisms, such as shafts, connecting rods, studs, gears, and others, widely used in manufacturing industries like machine tools, automobiles, and tractors. Especially for large components in heavy machinery manufacturing, quenching and tempering treatments are even more common. Therefore, quenching and tempering occupies a very important position in heat treatment processes. In mechanical products, the performance requirements for quenched and tempered parts vary depending on their specific loading conditions. Generally speaking, all quenched and tempered parts should possess excellent comprehensive mechanical properties—appropriately balancing high strength with high toughness—to ensure long-term smooth operation of the parts. Quenching and tempering typically refers to a heat treatment process that involves quenching followed by high-temperature tempering, resulting in the formation of tempered sorbite microstructure. The method involves first quenching at a temperature of Ac3+30~50℃ for hypoeutectoid steels, Ac1+30~50℃ for hypereutectoid steels, and slightly higher temperatures for alloy steels compared to carbon steels. After quenching, tempering is carried out at 500~650℃. Is quenching and tempering difficult? Actually, the simplest approach can often be the most challenging. Sometimes it’s precisely this balance between strict adherence to procedures and flexible adaptation that makes it so hard to master. Let’s take a look at what the experts on heat treatment forums have to say: First: Taking my experience in a professional heat treatment plant as an example, let me talk about the difficulty of quenching and tempering: 1. Many customers provide materials that are not accurately specified. Even now, delivery notes still state “carbon steel,” “cold-rolled sheet,” or “spring steel”—it’s truly embarrassing. Every time I encounter such products, I have no choice but to test sparks and trial tempering temperatures before proceeding. 2. Some parts have complex shapes and are close to critical dimensions. Coupled with equipment limitations at each heat treatment plant, if you choose an inappropriate quenching temperature or quenching medium, cracking or deformation may occur. If the quenching temperature is too high, cracks are more likely to form; if the holding time is improperly set, deformation will increase. If the quenching temperature is too low, to maintain hardness, the tempering temperature must be lowered. Although the hardness might meet specifications, the microstructural performance indicators won’t satisfy drawing requirements. 3. While quenching and tempering is indeed crucial, it’s only one step in the product manufacturing process. To ensure good quality, you must pay attention not only to the heat treatment itself but also to the preceding forging process and the subsequent machining operations. Otherwise, people from those stages will come looking for you—forgers will blame your tempering technique, machinists will say your quality isn’t up to par, and they’ll complain about how difficult it is to machine... 4. Some quenched and tempered products are used in wind power or extreme environments, where impact toughness is required at temperatures as low as -30 or -40 degrees Celsius. Honestly, the temperature, time, cooling, and tempering during quenching and tempering must reach the ideal state. And if the material has inclusions or segregation defects, quenching and tempering becomes even harder... Having worked in a professional heat treatment plant for nearly 10 years, I’ve concluded that whether it’s quenching and tempering or other quenching processes, as long as you put your heart into it and delve deeply into the details, achieving the drawing-specified quality for your products isn’t difficult. What’s really challenging is ensuring that every batch and every single part—from the initial sample to the final batch—maintains exactly the same quality level. That’s precisely the key difference between China’s heat treatment quality and that of other countries. Second: I think as long as the equipment meets the requirements—such as effective furnace size, furnace temperature uniformity, temperature control accuracy, quenching delay time, furnace heating rate, cooling rate, quenching fluid cooling speed (including aging level, impurities, and circulation), cleaning quality—and ideally features automated control, and the raw materials come with certificates of quality and incoming inspection reports, and workers strictly follow the process, then quenching and tempering becomes quite easy. Of course, the real skill lies in daily practice; when you’re doing it, it feels completely natural. Third: The difficulty of quenching and tempering lies in the heating temperature, holding time, cooling medium, the shape and size of the workpiece, existing equipment, technical requirements, and the execution of the process itself. Fourth: Theoretically, quenching and tempering is a process that combines quenching with high-temperature tempering to obtain tempered sorbite. If any one of these steps fails to meet the requirements, the quenching and tempering result won’t be ideal, making it indeed quite challenging. Strictly speaking, you not only need to meet the hardness requirements but also the metallographic structure requirements. If you only consider hardness without taking microstructural performance into account, the difficulty won’t be as great—but often, this leads to poor microstructures after quenching and tempering. Fifth: Quenching and tempering is indeed an important process, but to achieve the desired results, we must strictly control every stage of the process—including heating temperature, holding time, cooling medium, and tempering temperature! From my personal experience, I believe that true quality stability in production requires minimizing the influence of human factors!!! To ensure high-quality quenched and tempered parts, we should upgrade the quenching and tempering production line—relying entirely on equipment and processes to guarantee product quality! Sixth: As a pre-heat treatment, quenching and tempering offers considerable leeway in terms of deformation and oxidation. However, controlling the quality of quenching and tempering is extremely difficult! I think the main difficulties lie in: First, material instability—when one batch of material passes inspection while another batch fails, it’s a common problem. Second, external environmental influences—what you get in summer is different from what you get in winter (this is similar to normalizing, mainly due to the cooling medium). Third, equipment issues—since quenching and tempering is usually performed on blanks, both loading/unloading and operation, including furnace design, tend to be rather rough. Equipment imprecision and human carelessness often significantly affect product quality! Seventh: Take the crankshaft as an example—the difficulty of quenching and tempering. Due to the complex shape of the part, selecting the right quenching temperature and quenching medium becomes critical: If the quenching temperature is too high, cracks will appear; if it’s too low, the tempering temperature must be reduced. Although the hardness might meet specifications, the microstructural performance indicators won’t satisfy drawing requirements. If you also select the wrong material in the middle of the process, things become even more complicated. Quenching and tempering isn’t easy; tempering itself is full of subtleties. Keep detailed records and accumulate experience. Discussion: Common problems during quenching and tempering: 1. Material chemical composition—materials with the same grade may have different chemical compositions, requiring different quenching temperatures. 2. Workpiece shape—long, slender rod-shaped parts and thin-walled parts undergo significant thermal deformation, leading to substantial post-processing correction. 3. Part dimensions—take 45 steel as an example: the critical quenching size is 9–14 mm. Designers who don’t understand heat treatment often specify quenching and tempering after machining. They don’t care whether the part cracks during quenching or not; once it cracks, they’ll criticize your “level.” 4. Quenching and tempering of cast parts: some castings already have poor casting quality—porosity, looseness, inclusions, and other major casting defects. Once cracks appear after quenching, they’ll blame your heat treatment for causing the cracking. 5. Mixing materials—anyone involved in heat treatment has probably encountered this: carbon steel and alloy steel sent for heat treatment without being separated. Case 1: Material—40Cr; Type—conveyor chain plates; Quenching equipment—custom-made mobile quenching tank with air-cooled heat exchanger; Process flow—forging—residual heat quenching—tempering—machining—shipment. Reasoning: Initially, we considered that this product would undergo significant deformation during forging and would be produced on a friction press with a stable forging cycle. Also, there wouldn’t be major issues with grain size or hardness. Thus, we chose residual heat quenching after forging. Considering the relatively high quenching temperature and large thermal capacity, we initially used oil quenching, but the on-site environment and safety were difficult to control. So we switched to a water-based PAG quenching medium. Actual results: During final adjustments, hardness, metallographic structure, and grain size were all OK, but a large proportion of quenching cracks appeared—very regular, standard arc-shaped quenching cracks. We adjusted pre-cooling time, quenching temperature (tried from 900 to 780℃), cooling time, and quenching fluid concentration, but the problem remained unsolved. We were stumped. Finally, we had no choice but to switch back to oil quenching, which was 100% successful, with no changes in conditions. Conclusion: For forged parts with dramatic shape and interface changes, when performing residual heat quenching, use caution with quenching media and opt for media with slower cooling rates. This conclusion may not be entirely correct; there could still be some unresolved issues. If anyone is interested, feel free to help me figure it out. Case 2: 45 Steel Quenching and Tempering—45 steel quenching and tempering requires a hardness of 180–230. My metallographic structure was at Grade 1. I quenched at 840℃ in saltwater and tempered at 660℃. There were no issues with either structure or hardness, but the customer reported that machining was too difficult (tool sticking, rapid tool wear). Later, adopting subcritical quenching largely solved this problem. Experience: Several examples shared by online users—quenching and tempering in large continuous belt furnaces, where forgings were quenched and tempered without normalizing beforehand. The quenching holding temperature should be chosen at the lower limit; it’s best to adopt pre-cooling quenching. The quenching temperature holding time shouldn’t be too long, otherwise the martensite needles will become coarse. For products that have undergone spheroidizing annealing, the quenching holding temperature during tempering should be chosen at the upper limit; it’s best to quench at the same temperature as the holding temperature. The quenching heating time coefficient should be taken at the upper limit, and the holding time should be sufficient; otherwise, the tensile strength will be noticeably low... Experience: 40Cr material, 1.2 meters long, 300 mm in diameter, hollow, with a central hole around 260 mm and a wall thickness of about 40 mm. The requirement was quenching and tempering—only hardness was specified, with no other technical requirements. Equipment: Ordinary box furnace (old equipment, some parameters unclear) + pit furnace. Process: With over 10 years of experience, I haven’t handled many parts of this material. At the time, the senior technician just wanted to simply austenitize, quench, check the hardness, and adjust the tempering temperature accordingly to meet the hardness requirement. Since he wasn’t sure, he came to me for advice. I first listened to his idea: To prevent quenching cracks, directly quench in oil, check the hardness after quenching, and adjust the tempering temperature to meet the customer’s hardness requirement. I clearly stated at the time that this wouldn’t work. I boldly asked him: What exactly does quenching and tempering mean? (....) My suggestion at the time: Ensure sufficient holding time. After removing the part from the furnace, hook both ends with U-shaped hooks, lift them using a crane, and allow them to cool slowly in air for about 5–10 seconds (depending on color). Then quickly immerse them in water (5% alkaline solution), gently shaking the crane as a stirrer. When the temperature drops to around 200–300℃, quickly lift them into oil for cooling. Once cooled to the bottom, wipe off surface oil and quickly transfer them into the preheated pit furnace for tempering. After tempering, hardness testing showed that the parts fully met the customer’s requirements, and according to the customer, they were very satisfied with the performance. Experience Summary: 1. Material chemical composition—materials with the same grade may have different chemical compositions, requiring different quenching temperatures. 2. Workpiece shape—long, slender rod-shaped parts and thin-walled parts undergo significant thermal deformation, leading to substantial post-processing correction. 3. Part dimensions—take 45 steel as an example: the critical quenching size is 9–14 mm. Designers who don’t understand heat treatment often specify quenching and tempering after machining.They don’t care whether your part cracks after quenching or not; once it does crack, they’ll immediately criticize your “level of expertise.” 4. Quenching and tempering of cast parts: Some castings inherently have poor casting quality—porosity, looseness, inclusions, and a host of other casting defects. After quenching and cracking occurs, they’ll blame you for the cracking during heat treatment. 5. Mixing of materials—those of us involved in heat treatment have all encountered this: carbon steel and alloy steel are sent for heat treatment without being properly separated. 6. Equipment issues: During tempering, if the equipment experiences “temperature drift” and the temperature rises, the hardness after quenching and tempering will be low, forcing you to re-quench and start all over again. Experience and shape effects: From the perspective of workpiece geometry, spherical workpieces have the strongest ability to withstand quenching; next come bar-shaped parts. Plate-like parts have the weakest ability to withstand quenching. Disc-shaped parts should be considered similar in shape to plates, and thus also have poor quenching performance. Even under relatively ideal quenching conditions—such as a sufficiently large cooling tank with ample coolant volume ensuring minimal temperature fluctuations in the quenching bath—the peripheral hardness of such large disc-shaped parts might reach around 50 HRC. However, in the large area from the geometric center to the periphery, the hardness will inevitably be both uneven and low. Discussion: What kind of equipment is best for quenching and tempering? 1st floor: Multi-purpose furnaces are commonly used for quenching and tempering. However, the quenching delay time can’t be precisely controlled, and the process is too slow. If the batch size is large and the product dimensions meet the requirements, a continuous furnace might be better. It’s crucial to carefully control the time between出炉 (removal from the furnace) and quenching. 2nd floor: By “delay time,” you mean the period from the rear chamber to the front chamber and then to immersion in the oil tank—isn’t that right? This issue indeed exists, especially for small, thin parts with low packing density. 3rd floor: Although multi-purpose furnaces do have transfer times, these are much shorter compared to井式炉 (well-type furnaces). Parts remain free from oxidation and decarburization, and the cooling effect is significantly better. Part consistency is very high! The quality is quite stable, though limited by the cooling medium. However, quenching and tempering in multi-purpose furnaces also comes with higher costs. 4th floor: For large parts, multi-purpose furnaces perform well—they offer protection, preventing oxidation and decarburization! For small parts, I think mesh belt furnaces are a good choice: for example, parts less than 40 mm long and not exceeding the width of the mesh belt. They have lower costs (compared to multi-purpose furnaces), higher production output, stable quality, and lower labor intensity for workers. 5th floor: Quenching and tempering in multi-purpose furnaces—same heating temperatures as conventional furnaces. The advantage is that the protective atmosphere during heating and quenching shields the workpieces from oxidation and decarburization. Under normal circumstances, products are less likely to oxidize or decarburize. The carbon potential of the furnace gas can be set at 0.5. If the carbon potential is too high, carbon deposits easily build up inside the furnace, which is wasteful. If the carbon potential is too low, it affects the carburizing results of the next batch. 6. Equipment issues: During tempering, if the equipment experiences “temperature drift” and the temperature rises, the hardness after quenching and tempering will be low, forcing you to re-quench and start all over again.
Thank you for your interest in our product line. Please leave us a message, and we’ll get back to you as soon as possible!
