Surfacing valve sealing surface technology (Tech Center of Shanghai HY Industry Co., Ltd)

Surfacing and spray welding as a hard surface processing technology, widely used in the field of valve manufacturing, surfacing and spray welding technology advances, but also makes the valve better meet the increasing environmental requirements of wear resistance, corrosion resistance, oxidation, etc., and surfacing and spray welding parts as difficult to cut materials had limited the valve manufacturing industry processing efficiency and cost, and with the development of surfacing technology and cutting technology, new materials, new technologies and new processes With the development of surfacing technology and cutting technology, new materials, new technologies and new processes, Surfacing valve sealing surface technology has been applied to promote the progress of valve manufacturing technology and the development of new surfacing and spray welding technology.

With the development of modern technology, more and more valves in high temperature, high pressure and high corrosion, high wear resistance and other environmental work, in order to improve the valve wear resistance, high temperature and corrosion resistance and other properties, the factory now commonly used surfacing, spray welding technology to increase the hardness of the valve shell or internal parts to improve the wear resistance and service life of the valve. The so-called overlay welding, spray welding is in the workpiece surface overlay welding or spray welding a special layer of wear-resistant, corrosion-resistant, anti-oxidation materials. The workpiece has sufficient hardness, wear resistance and corrosion resistance to meet the requirements of the valve, and is simple to operate. However, if you want to get the valve surfacing to meet the design and use requirements, you must strictly follow the development of work instructions and operational requirements, and at the same time, according to the base material (workpiece material) and welding method to select the appropriate welding material.

At present, the commonly used Surfacing valve sealing surface alloys are iron-based, nickel-based, cobalt-based, nickel-based and other surfacing materials. Among them, cobalt-based alloys have good high-temperature performance, excellent thermal strength, corrosion resistance and heat-resistant fatigue properties, than iron-based, nickel-based alloy wear resistance, corrosion resistance is good. Therefore, in valve development and manufacturing, cobalt-based cemented carbide is widely used in the overlay welding of various valve housings and sealing surfaces with its irreplaceable excellent comprehensive performance. At present, the commonly used valve surfacing techniques include oxy-acetylene flame surfacing, manual electric arc welding and plasma arc welding, etc.

Shanghai HY Industry Co., Ltd on the above commonly used three surfacing technology to understand the valve surfacing analysis, and for surfacing (spray welding) valve sealing surface of high hardness cutting was discussed.

First, oxy-acetylene flame surfacing valves

• Oxy-acetylene flame temperature is low (about 3100 ℃), and can adjust the energy rate of the flame, you can get a low dilution rate (1% ~ 10%) and a thin overlay layer, using oxy-acetylene surfacing valve sealing surface, often using carbonized flame (also known as reduction flame) surfacing valve. And depending on the base material, the need for preheating before welding and post-welding heat treatment. Generally can be used 300-600 ℃ preheating. After welding, the workpiece should be tempered at 600-700℃ for 1 hour and then slowly cooled or immediately put into a dry and hot sandbox or slow cooling in grass ash to avoid cracking. In order to minimize the dilution rate during the overlay welding process, the preheating temperature, the interlayer temperature should not be too high. Used in the temperature below 650 ℃, pressure less than 25Mpa, the medium is water, superheated steam, oil and other high parameters of the valve sealing surface overlay welding. Oxy-acetylene flame surfacing is not only used for surfacing, but also applied to spraying, spray welding and other processes. Oxy-acetylene flame surfacing (also known as spray welding) is a flexible application and material-saving surfacing process method. DN (DN is the nominal diameter (or nominal diameter), is a variety of common caliber of the pipe and pipeline accessories) ≤ 80mm valve sealing surface overlay welding is particularly suitable, only one layer of overlay welding, after processing more than 0.5mm to meet the requirements. The hardness of the surfacing is also different, cobalt-based alloy material Stellite No. 12 hardness after surfacing HRC48, Stellite No. 6 hardness after surfacing HRC42.

Second, manual arc welding surfacing valve

• Manual arc welding overlay valve sealing surface is a relatively common process method, which is characterized by simple equipment and process, easy to operate, flexible selection of overlay welding rod according to needs. Due to the high dilution rate of manual arc welding, it will lead to a decrease in the hardness and wear resistance of the overlay layer, if you want to obtain a dilution-free surface working layer at least 4 layers of overlay welding, after processing the thickness of the weld layer of 6mm or more. Because of the many types of overlay welding rod, its characteristics vary, so you can choose the appropriate overlay welding rod and overlay layer thickness according to the requirements of the parts. For the corrosion-resistant requirements of the structure, should choose the corresponding matching special welding rod or the chemical composition of the deposited metal and its similar welding rod; complex structure, rigid, poor welding conditions, harsh working requirements of the important structure, should be selected with low-hydrogen alkaline welding rod. Different welding electrodes surfacing out of the valve hardness is also different, high-chromium cast iron surfacing electrodes hardness up to HRC50 or more, tungsten carbide surfacing electrodes surfacing out of the hardness up to HRC60 or more.

Third, plasma arc welding overlay valve

• Plasma arc is a highly efficient heat source, its arc zone temperature can reach 20000K, its gentle arc flame characteristics can obtain shallow and uniform depth of melt, is the ideal heat source for overlay welding, and therefore is widely used for valve sealing surface overlay welding. According to the characteristics of plasma arc welding can be overlay welding high hardness, high strength, especially high melting point, high strength alloy powder that can not be welded by other welding methods. Material-wise, plasma arc welding mainly uses powder materials. Compared with manual arc welding and oxy-acetylene gas welding and other surfacing methods not only high production efficiency, stable quality, easy to mechanize and automate; and the welding method uses powder as filler material, to overcome the carbide is difficult to make wire and other problems. And China’s powder manufacturers, product series can basically meet the requirements of various applications, especially in the valve, the current powder has been more mature. More suitable for the overlay welding of iron-based alloy materials.

• Plasma arc welding overlay valve process requirements: before welding requires mechanical cleaning of the workpiece processing, the surface of the workpiece must have a metallic luster, remove rust, oil and oxides. Certain workpieces need to be preheated before overlay welding, such as overlay cobalt-based powder nominal size greater than DN25 carbon steel welding needs to be preheated, for overlay iron-based powder, when the selected powder species and the substrate material preheating requirements are required to preheat, for pearlite, martensitic steel, even smaller parts must be preheated to prevent the appearance of the overlay layer cracking. Preheating temperature according to the base material. Batch overlay parts should be preheated in the furnace. Preheating temperature time according to the size and shape of the workpiece. The workpiece should be heat treated immediately after surfacing; the use of cobalt-based powder surfacing carbon steel workpiece, nominal size less than DN25, without stress relief heat treatment; the use of iron-based powder surfacing, when the selected powder and the base material has heat treatment requirements, the corresponding heat treatment is required.

Fourth, the processing of Surfacing valve sealing surface tool material

After overlay welding, spray welding method to make the valve surface performance has been significantly improved, the sealing surface sealing better, more wear-resistant, long service life, adapt to a variety of high temperature and pressure, high corrosion of the working environment. Valve according to the different working environment, the hardness of the valve overlay layer is also different, so the problem arises when cutting and processing the overlay valve, due to the high hardness of the overlay valve difficult to process, the tool does not wear-resistant phenomenon, not only can not improve the processing efficiency, and increase the cost of tools. So the selection of the correct tool material is an important factor in achieving high efficiency and low cost. At present, the tool materials for processing valve overlay are carbide tool, ceramic tool and non-metal binder cubic boron nitride tool.

• choose cemented carbide tools to process overlay valve sealing surface: the hardness of the carbide tool body itself is between HRC71-76, processing high hardness overlay valve sealing surface will not wear phenomenon, such as processing HRC40 below the valve sealing surface carbide tools are more economical, rough machining can choose YG8, etc., finishing options such as YT15.

• choose ceramic tools to process welded valve sealing surface: ceramic tool hardness is higher than carbide tools, the tool body itself hardness between HRC95-100, suitable for processing HRC45-55 between the welded valve sealing surface, but due to the brittle, limited by its impact resistance can only be used for finishing, eating tool depth is generally controlled within 0.3mm.

• choose cubic boron nitride tool processing welded valve sealing surface: cubic boron nitride tool hardness are higher than carbide tools and ceramic tools, due to the high hardness, and diamond tools collectively known as super-hard tools, suitable for processing hardness HRC45 above the welded valve sealing surface, but the biggest disadvantage of cubic boron nitride tool and ceramic tools as brittle, the effect of processing high hardness welded valve sealing surface is also not very obvious. Until the development of a non-metallic binder cubic boron nitride tool BN-S20 grade, to ensure the hardness of cubic boron nitride inserts at the same time, increase its impact toughness and high temperature resistance, because BN-S20 grade belongs to the non-metallic binder cubic boron nitride tool, compared with the traditional cubic boron nitride tool increased toughness, not only high hardness, and excellent wear resistance and impact resistance, eating tool depth can be The depth of draft can reach 7.5mm, roughing and finishing are available. It is very suitable for machining the sealing surface of overlay valves made of iron-based and cobalt-based alloys above HRC45.

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