Good use of heat treatment equipment is undoubtedly a very important thing for both the user of the equipment and the manufacturer of the equipment. However, another key content of using equipment well is how to maintain, repair and maintain equipment scientifically.

Traditional equipment maintenance, repair and maintenance are limited to electromechanical repairs. It is a kind of maintenance, repair and maintenance. New equipment maintenance, repair and maintenance are based on the concept of being responsible for the heat treatment process of the product and the quality of the heat treatment of the product. At the same time, with the continuous popularization of heat treatment equipment "less people or unattended mode".

This requires the replacement of traditional maintenance personnel who are about to be eliminated with highly professional new maintenance (including maintenance and maintenance, etc.) engineers. They must have: the ability to pre-diagnose heat treatment equipment failures, the ability to promptly and quickly eliminate heat treatment equipment failures, the ability to ensure the reliability and stability of heat treatment equipment troubleshooting, the ability to evaluate the necessity of heat treatment equipment maintenance,...

Although the job of training such a new type of maintenance engineer is extremely severe and difficult, how to retain such a new type of maintenance engineer is even more severe and difficult. At present, not only the military industry companies are "at a loss" about this, even the famous foreign-owned enterprises are also "at nothing"! The essential question is: how to recognize its value? How to recognize their own value? How to evaluate this value scientifically? ...

In these respects, we do have a big gap. However, instead of endlessly arguing with each other about "this difference in value", it is better to work hard to create a scientific evaluation system, that is, to fully recognize the social value of new maintenance engineers, use the social market, and integrate the company itself. The evaluation ability of the company has enabled the continuous and healthy development of the new maintenance engineer team, enabling the company’s heat treatment equipment to be scientifically repaired, maintained and maintained.

With the continuous updating of the overall situation of military industry enterprises, the situation of the heat treatment industry has also been significantly improved. Vacuum heat treatment equipment, multi-function (carburizing, nitriding, etc.) furnace equipment, ion nitriding equipment, automatically controlled high and intermediate frequency equipment, precisely controlled cooling equipment, dual-circulation cooling water systems, etc. have been widely used. , And gradually replace the traditional non-energy-saving environmental protection old box-type resistance furnace. As far as parts are concerned, the proportion of this replacement is very large. The newly updated heat treatment equipment is usually very advanced, especially the imported heat treatment equipment has basically realized the "small or unattended operation mode", and has fully utilized the reliability, stability and consistency of the equipment itself to replace the inconsistency of personnel. Reliability is unstable and inconsistent.

Thus, the reliability, stability and consistency of the heat treatment quality of materials and parts are guaranteed. This will inevitably impose extremely high requirements on the overall quality of such heat treatment equipment operators, including: metal materials and heat treatment knowledge, mechatronics knowledge, computer knowledge, vacuum acquisition knowledge, test and measurement knowledge, etc. Knowledge and understanding of mutual coupling. However, the current education, employment, training and other personnel training systems are indeed very contradictory to the current speed of construction of conditions in the heat treatment industry of military enterprises. Due to the lack of relevant support measures for skilled positions, there is not only a shortage of personnel, but also extremely unstable. The brain drain is extremely serious, which is also a "common problem" in the entire manufacturing industry.

Based on the above situation, the heat treatment still follows the traditional model, namely: "wear new shoes and follow the old road". Most companies are still arranging most of the personnel to monitor the furnace. It is nothing more than installing the furnace, releasing the furnace, recording some artificially set tables... This model can only solve the employment problem, and it has systematically hindered the training of employee skills, the development of new products, and the improvement of product quality and stability. Therefore, no matter how the equipment reaches the production site, it can actually be solidified into several basic operating modes, most of the functions are not used (this is the same as purchasing in the initial stage of conditional construction, pursuit of complete functions, higher levels and new quality When the equipment is the same is indeed "opposite to each other"). Therefore, it is easy to understand that the rate of improvement of product quality in the entire heat treatment industry is not proportional to the rate of increase of new equipment that is conditionally constructed. To solve the above problems, the core is not to solve the equipment problem, but to solve the personnel problem. People’s responsibility must extend from "observing the furnace" to "guaranteeing the final performance of the material and ensuring the installed service performance of the product."

The medium and high temperature equipment in oil refining, large thermal power units, and pressurized gasification units for syngas production from coal instead of oil all require steel to have good oxidation resistance (heat resistance), sufficient high temperature strength and excellent toughness. , Various types of chromium molybdenum steel are widely used.

Through some experimental studies and engineering examples, the impact of pressure-bearing equipment made of chromium-molybdenum steel on the properties of steel after overall heat treatment is introduced. For this reason, the overall post-weld heat treatment process of chromium-molybdenum steel equipment was discussed and some suggestions were put forward.

At present, some large and medium-sized manufacturing plants in my country have more steel grades for manufacturing such equipment: 1Cr05Mo (for example: 15CrMoR, 15CrMog, SA387Grl2, etc.); of course, there are also Cr-Mo steels containing vanadium. In addition, some oil refining equipment is made of explosive clad steel plate, the base layer is Cr-Mo steel, and the clad layer is stainless steel.

l Post-weld overall heat treatment of Cr-Mo steel (PWHT or SR treatment)

Cr-Mo steel equipment requires overall post-weld heat treatment to eliminate welding residual stress, reduce the hardness of the welded joint and improve its mechanical properties. With the increase in the strength or thickness of the large-scale steel of the device, the manufacturing process specifications may increase the heat treatment temperature or extend the holding time.

The Cr-Mo steel weld metal contains more hardened structure. After a higher temperature overall heat treatment, the toughness is improved while eliminating stress and reducing hardness. However, increasing the overall heat treatment temperature or excessively extending the holding time (that is, increasing the tempering parameter) will cause carbides in the metallographic structure of the weld metal and heat-affected zone to accumulate along the grain boundaries, and may also cause the ferrite grains to be coarse化. The strength of the welded joint is reduced, the toughness becomes worse, and stress embrittlement (reheat embrittlement) occurs.

The brittle transition temperature of the weld metal is the lowest (the highest toughness); the brittle transition temperature of the three weld metals has increased in different degrees (the toughness becomes worse, that is, reheat embrittlement). The reheat embrittlement tendency of 25Cr-1Mo is very slight. When it is subjected to higher temperature and longer heat treatment, the curve rises less obviously, and the toughness remains at a higher level. This is in line with the Cr, The high content of Mo is consistent with the performance of strong heat resistance; 25Cr-0.5Mo has an obvious tendency to reheat embrittlement. When the T value starts from about 19.5, the curve rises significantly, which is not high toughness. There has been a sharp drop. However, since the vTrs value of the weld metal of the 1.25Cr-0.5Mo steel is almost the same before and after the step-cooling test, it shows that its temper brittleness is very small. For this type of steel, the content of C, Si, Mn is generally increased or adjusted. Improve hardenability and reduce oxygen content to increase strength and toughness, instead of improving tempering parameters to achieve the goal.

Explosive stainless steel clad steel plate vessel heat treatment Explosive stainless steel clad steel plate is used more and more widely in the pressure vessel industry due to its excellent corrosion resistance, perfect combination of mechanical strength and reasonable cost performance. However, the heat treatment problem of this material It should also attract the attention of pressure vessel designers. Pressure vessel designers usually pay more attention to the technical index of the composite plate is its bonding rate, and the heat treatment of the composite plate is often considered very little or thinks that this problem should be considered by the relevant technical standards and manufacturers. The process of explosive processing of metal composite panels is essentially a process of applying energy to the metal surface. Under the action of high-speed pulses, the composite material obliquely collides with the substrate. In the state of the metal jet, a zigzag composite interface is formed between the composite metal and the base metal to achieve interatomic bonding. The base metal after explosive processing has actually undergone a strain-strengthening process. As a result, the tensile strength σb increases, the plastic index decreases, and the yield strength value σs is not obvious. Whether it is Q235 series steel or 16MnR, after explosive processing and then testing its mechanical performance indicators, it shows the above-mentioned strain strengthening phenomenon.

In this regard, both the titanium-steel composite plate and the nickel-steel composite plate require that the composite plate be subjected to stress relief heat treatment after explosive compounding. The 99 edition "Capacity Regulations" also has clear regulations on this, but there is no such regulation for explosive composite austenitic stainless steel plates. In the current relevant technical standards, the question of whether and how to heat the austenitic stainless steel plate after explosive processing is more ambiguous. GB8165-87 "Stainless steel clad steel plate" stipulates: "According to the agreement between the supplier and the demander, it can also be delivered in hot rolled or heat-treated state." It can be supplied by leveling, trimming or cutting. According to the requirements of the buyer, the composite surface can be pickled, passivated or polished, and can also be supplied under heat treatment." There is no mention of how to perform heat treatment. The main reason for this situation is still the aforementioned austenitic stainless steel producing intergranular corrosion sensitized area problem. GB8547-87 "Titanium-steel composite plate" stipulates that the heat treatment system of stress-relieving heat treatment of titanium-steel composite plate is: 540℃±25℃, heat preservation for 3 hours. And this temperature is just within the sensitization temperature range of austenitic stainless steel (400℃-850℃).

Therefore, it is more difficult to give clear regulations on the heat treatment of explosive composite austenitic stainless steel plates. In this regard, our pressure vessel designers must have a clear understanding, give full attention, and take corresponding measures. First of all, 1Cr18Ni9Ti should not be used for composite stainless steel, because compared with low-carbon austenitic stainless steel 0Cr18Ni9, it has a higher carbon content and is more prone to sensitization, which reduces its resistance to intergranular corrosion. In addition, when the shell and head of the pressure vessel made of explosive composite austenitic stainless steel plate are used in harsher conditions, such as high pressure, pressure fluctuations, and extremely hazardous media, 00Cr17Ni14Mo2 should be used. Ultra-low carbon austenitic stainless steel can minimize the possibility of sensitization. And should clearly put forward the heat treatment requirements of the composite board, and negotiate with the relevant parties to determine the heat treatment system, so as to achieve a certain amount of plastic storage for the base material and the composite material to have the required corrosion resistance.

Does austenitic stainless steel pressure vessel require post-weld heat treatment? Post-weld heat treatment utilizes the reduction of the yield limit of metal materials at high temperatures to produce plastic flow in places with high stresses, thereby eliminating welding residual stress and improving welding The plasticity and toughness of the joint and heat-affected zone improve the resistance to stress corrosion. This stress relief method is widely used in pressure vessels made of carbon steel and low alloy steel with a body-centered cubic crystal structure. The crystal structure of austenitic stainless steel is face-centered cubic. Because metal materials with a face-centered cubic crystal structure have more slip surfaces than body-centered cubic, they exhibit good toughness and strain strengthening properties. In addition, in the design of pressure vessels, stainless steel is often used for the two purposes of anti-corrosion and meeting the special requirements of temperature. In addition, stainless steel is more expensive than carbon steel and low alloy steel, so its wall thickness is not very high. thick. Therefore, considering the safety of normal operation, there is no need to put forward post-weld heat treatment requirements for pressure vessels made of austenitic stainless steel.

As for corrosion due to use, material instability, such as fatigue, impact load and other abnormal operating conditions, it is difficult to consider in conventional design. If these conditions exist, relevant scientific and technical personnel (such as design, use, scientific research and other relevant units) need to conduct in-depth research and comparison experiments to come up with a practical heat treatment plan and ensure that the comprehensive performance of the pressure vessel is not affected. Otherwise, if the need and possibility of heat treatment for austenitic stainless steel pressure vessels is not fully considered, it is often not feasible to simply compare the conditions of carbon steel and low alloy steel and put forward heat treatment requirements for austenitic stainless steel. In the current standards, the requirements for post-weld heat treatment for pressure vessels made of austenitic stainless steel are relatively vague.

It is stipulated in GB150-89 "Steel Pressure Vessel" 10.4.1.3. "Except as otherwise specified in the drawing, the cold-formed austenitic stainless steel head may not be heat treated". As to whether heat treatment should be carried out in other cases, it may vary depending on the understanding of different people. It is stipulated in GB150-1998 "Steel Pressure Vessels" 10.4.1 that the vessels and their pressure components that meet one of the following conditions shall be heat treated. The second and third items are: "containers with stress corrosion, such as containers containing liquefied petroleum gas, liquid ammonia, etc." and "containers containing extremely toxic or highly hazardous media". It is only stipulated in 10.4.1.1.f): "Except as otherwise specified in the drawing, the welded joint of austenitic stainless steel may not be heat treated". Analyzing from the level of standard expression, this requirement should be understood as mainly for the various situations listed in the first item.

The second and third items mentioned above may not be included. Therefore, it is recommended that when appropriate, "10.4.1.1.f)" should be replaced by "10.4.1.4" in the form of "addition". In this way, the post-weld heat treatment requirements for austenitic stainless steel pressure vessels can be expressed more comprehensively and accurately, so that the designer can decide by himself whether or not and how to heat the austenitic stainless steel pressure vessels according to the actual situation. Article 74 of the 99 edition of the "Capacity Regulations" clearly states: "Austenitic stainless steel or non-ferrous metal pressure vessels generally do not require heat treatment after welding. If heat treatment is required for special requirements, it should be noted on the drawing."

Annealing: Usually refers to the treatment required to make the material soft and stress-free. For most materials, such as carbon steel, this would mean raising the temperature very high and then cooling very slowly to room temperature.
Quenching annealing: The purpose here is to obtain a "soft" structure again, but slow cooling will adversely affect the material. Typical is 300 series stainless steel. (Also called austenitic stainless steel.) These stainless steels will not undergo any significant phase change in the main body of the material during the heat treatment process, but if they are kept at a certain intermediate temperature, it may lead to the formation of local harmful phases. Or particles. The range of the long time period. To prevent this from happening, the material is rapidly cooled (quenched) from high temperature. (Usually 1050°C), which reduces the residence time of the material in the temperature range of 500 – 850°C. In this temperature range, they will experience the formation of grain boundary carbides, thereby severely reducing the corrosion resistance of the material .
Solution treatment: The main purpose of heat treatment is to ensure that all different alloying elements are uniformly dispersed throughout the material and "dissolved" in the material as much as possible. This is usually performed on castings, because the solidification process during the casting process tends to cause the material to have relatively large differences in certain alloying elements in different parts of the structure. There are often areas with high concentrations of certain elements and low concentrations of other elements. By raising the temperature to the point where a large amount of diffusion occurs, these uneven alloying element concentrations will become uniform. At these high temperatures, certain phases (such as carbides) will also be "dissolved" (dissolved) by the material. In order to retain as many alloy elements as possible in the solution, some materials are usually quenched after solution treatment. Therefore, this is very similar to the quench annealing discussed above.
Quench hardening: In order to make certain materials (such as carbon steel and low alloy steel) have higher hardness, the material can be heated to a certain temperature above which phase change occurs in the material. (For carbon steel, it is usually 950°C.) The material is then rapidly cooled (quenched) to form some metastable phases (such as martensite), resulting in a higher hardness of the material. When materials are hardened by quenching, they are usually brittle.
Tempering: To soften the material that has been hardened in the previous thermal cycle (for example, quench hardening), you can raise the temperature of the material again to a temperature below which the material begins to undergo a bulk phase change (usually heating To 650-700) °C) and leave it there for a while. During this tempering cycle, the hardened martensite is transformed into tempered martensite, which is not as hard as quenched martensite, but is still very hard.
Quenching and tempering: This is a combination of the above two heat treatment cycles.
Relieve stress: When ductile metals are plastically deformed, they will eventually generate a lot of residual stress in the material. Welding also creates these residual stresses around the weld seam. By increasing the temperature of the metal, the yield strength of the metal decreases. (Yield strength is the stress at which a material begins to plastically deform.) When the yield strength drops below the residual stress level due to high temperatures, the material will "relax". This reduces the stress trapped in the material by deformation or welding activity. Carbon steel can usually relieve stress at a temperature of about 600°C. At this temperature, the residual stress is usually reduced to about 30% of the yield strength of the material at room temperature. The main reason for stress relief is that it improves the fracture toughness of the component. It also reduces the possibility of certain corrosion mechanisms (such as stress corrosion cracking).