Common valve steel castings defects repair welding and repair welding

1, Overview In the industrial pipeline pressure valve, the cast steel valve due to its cost economy and design flexibility, are widely used. However, due to the casting process by casting size, wall thickness, climate, raw materials and construction operations of the various constraints, the casting will appear trachoma, porosity, cracks, shrinkage, shrinkage and inclusions and other casting defects, especially sand casting alloy Steel castings for more. Because the more alloying elements in the molten steel liquid flow worse, casting defects are more likely to produce. Therefore, it has been a common concern that hot and cold processing of valves should be concerned with determining defects and formulating a reasonable, economical, practical and reliable welding process to ensure that the repaired valves comply with the quality requirements. This article describes several common defects of steel castings repair methods and experience (electrode using the old brand said). 2, defect processing 2.1, defect judgment In the production practice, some casting defects do not allow repair welding, such as through the cracks, penetrating defects (wear the bottom), honeycomb-shaped pores, can not remove the sand and slag inclusions over 65cm2 Shrinkage, etc., as well as the other contract can not be repaired in the major shortcomings. Before welding repair should determine the type of defect. 2.2, defect elimination In the factory can generally use carbon arc gouging blow to cast defects, and then use the portable angle grinder to polish the defect area to reveal the metallic luster. However, production practice is more direct use of carbon steel electrodes to remove large current defects and grind out the metal luster with a grinder. Casting defects in general can be used <4mm-J422 electrode, 160 ~ 180A current, the defects in addition to clean, grinder grind the defect into a U-shaped, reducing welding stress. Complete removal of defects, welding quality good. 2.3, the defective parts of preheated carbon steel and austenitic stainless steel castings, where repair welding area of ​​<65cm2, the depth of <casting thickness of 20% or 25mm, generally without preheating. However, pearlite steel castings such as ZG15Cr1Mo1V and ZGCr5Mo should be preheated due to the tendency of steel to harden and tend to be cracked. The preheating temperature is 200-400 ° C (welding with stainless steel electrode and the temperature is small) Holding time should not be less than 60min. If the casting can not be preheated, available oxygen - acetylene in the defect site and extended to 20mm and heated to 300-350 ℃ (back and dark visual observation dark red), large torch neutral flame gun at the defect and the surrounding do Swing the circle quickly for a few minutes and then move it slowly for 10 minutes (depending on the thickness of the defect), allowing the defect to warm up enough for quick repair. 3, repair welding 3.1, the requirements of austenitic stainless steel castings welding repair, in the ventilation, so that the rapid cooling. The pearlite low alloy steel castings and welding area is too large carbon steel castings should be selected leeward or blocked with a windshield to avoid cracks caused by fast cooling. Repair a pile of welding, welding residue should be immediately removed after the residue, and along the center of the uniform outward hammer, reducing the welding stress. If the repair welding points in several layers (usually 3 ~ 4mm for a welding layer), after each repair welding should be promptly removed dregs and hammer repair welding area. Such as welding in winter, ZG15Cr1Mo1V pearlite alloy steel castings, each layer also apply a layer of oxygen - acetylene repeated heating, and then quickly repair welding, in order to avoid welding cracks. 3.2, welding rod repair welding, welding rod should first check whether the preheating, the general electrode should be 150 ~ 250 ℃ drying 1h. After preheating the electrode should be placed in the incubator, so do with the use. Electrode preheating 3 times repeatedly, if the electrode surface coating off, cracking and rust, should not be used. 3.3, the number of repair welding Die casting, such as the valve shell by pressure test seepage, the same site generally only allow a welding, welding can not be repeated, because the multiple welding will make the steel coarse, affecting the casting pressure Performance, unless castings can be re-heat treated after welding. Other non-pressure-welded parts of the same site, the general provisions of welding no more than 3 times. The same part of the weld over the second carbon steel castings, welding should be done to eliminate stress treatment. 3.4, make up the height of the weld The height of the weld of the castings is generally about 2mm above the casting plane, to facilitate machining. Repair welding layer is too low, easily exposed after welding scars. Repair welding layer is too high, time-consuming effort materials. 4, repair welding 4.1, the important repair welding pressure test leaky castings, welding area> 65cm2 castings, depth> 20% of the casting wall thickness or 25mm cast, ASTMA217 / A217M-2007 are considered important Repair welding. For such an important repair welding A217 standards, should be carried out to stress treatment or completely reheat treatment, and this stress-relieving treatment or complete reheating treatment, must be approved by a qualified method, that is important to make welding Repair welding process. ASTMA352 / A352M2006 provides that the important repaired after stress relief or post-weld heat treatment is mandatory. And A217 / A217M corresponding to China's industry standard JB / T5263-2005 will be important in the definition of welding "heavy defects." However, in fact, except that the cast blank can be completely reheated, many defects are often found during finishing and are no longer completely heat-treated. Therefore, in practice, the welder, who usually holds the certificate of pressure vessel welding, is usually solved in the field with effective methods. 4.2, eliminating stress Finishing defects found After welding, has been unable to do the overall stress relief tempering, the general can use the defective parts of oxygen - acetylene flame local heating and tempering method. Large torch neutral flame back and forth slowly swing back and forth, the casting is heated to the surface appears visible dark red (about 740 ℃), heat (2min / mm, but not less than 30min). Asbestos board should be covered in the defect immediately after stress relieving treatment. Pearlescent steel valve on the path of defects, repair welding should also be filled in the diameter of the asbestos lumen plate, so slow cooling. Such operations, both simple and economical, but require a certain amount of practical experience welders. Stainless steel castings generally do not make repairs after welding, but should be in the ventilation Department of welding, so that the welding area fast cold. Unless after welding shows that has caused changes in austenitic tissue, or heavy defects. Under contractual and contractual conditions, the solution should be redone. Defective areas are too large and too deep carbon steel castings and a variety of pearlite castings, in the casting stage and although the roughing into the rough, but leave the finishing allowance, should be carried out after welding repair stress relief treatment. Carbon steel stress relief tempering temperature can be set to 600 ~ 650 ℃, ZG15Cr1Mo1V and ZGCr5Mo tempering temperature can be set to 700 ~ 740 ℃, ZG35CrMo tempering temperature is set to 500 ~ 550 ℃. All steel castings, the stress-tempering of the holding time are not less than 120min, and with the furnace cooled to below 100 ℃ baked. 4.3 Non-destructive testing For "heavy defects" and "important repairs" of valve castings, ASTM standard ASTMA217A217M-2007 stipulates that if the production of castings complies with the supplementary requirements of S4 (magnetic particle inspection), the repairs shall be made by the same quality standard Magnetic particle inspection to check. For castings complying with the requirements of S5 (Radiographic Inspection) Supplementary Requirements, castings seeped in hydraulics tests, or castings of any pit depth not exceeding 20% ​​of the wall thickness or 1in1 (25mm) Of any pit area of ​​about more than 10in2 (65cm2) of the weld repair, should be used to inspect the casting of the same standard of inspection for inspection. JB / T5263-2005 standard provides that after the flaw repair welding should be carried out by ray or ultrasonic testing. That is, for heavy defects and important repair welding, must be effective non-destructive inspection, proved to be qualified before use. 4.4, grade assessment For non-destructive testing of defects in the welding area report level, JB / T3595-2002 provides that, for power station valve steel casting valve bevel and repair parts should be GB / T5677-1985 assessment, three qualified . Valve butt welds shall be assessed according to GB / T3323-1987, two qualified. JB / T644-2008 in the casting of the same time, there are two different levels of defects also made clear that in the assessment of the same time there are two or more than two categories and different grades of defects, whichever is the lowest level as the comprehensive rating . When there are two or more than two kinds of the same grade and the same defects, the comprehensive level should be reduced by one. For welding slag defects of slag, unmelted and not welded, JB / T6440-2008 provisions can be seen as casting defects to assess the slag, weld defects of the pores can be seen as casting defects porosity assessment . The general condition valve order contract does not mark the valve casting grade, less qualified in the contract indicate the defect after welding, which often brings many contradictions to the valve production, inspection and sales. According to the actual quality level of steel castings and many years of experience in our country, the grade of the welding repair area is generally considered to be not lower than the third grade specified in GB / T5677-1985, ie the grade Ⅲ specified in the ASMEE446b standard. Acid-resistant corrosion-resistant pipeline conditions and high pressure cast steel valve shell pressure parts, should generally meet ASMEE446b Ⅱ level or above standards. The results of the radiographic examination showed that the defect generated during the welding process was less than the castings themselves and the grade was higher by the defect areas conforming to standard procedures and normative welding repairs. In short, repair welding as part of the manufacturing process, can not be taken lightly. 4.5, hardness testing Welding repair area although qualified by non-destructive testing, but for machining, it should check the hardness of welding area, which is to eliminate the effect of stress inspection. If the tempering temperature is not enough, or the lack of time, will lead to the welding area of ​​the deposited metal strength, poor plasticity, machining welding area will be hard, easily lead to tool cracking. Base metal and deposited metal inconsistent performance, but also easily lead to local stress concentration, there are obvious signs of welding junction transition. Therefore, the welding area need to use the hardness value to identify and detect. Gently grinding with a hand-held grinder repair welding area, the use of portable Brinell hardness hammer three points, the welding hardness and steel castings their hardness value comparison. If the hardness of the two regions are similar, then the basic success of oxy-acetylene tempering. If the repair welding hardness greater than 20 steel castings, it is recommended rework until the hardness close to the base metal. The hardness of the pressure-cast steel after heat treatment is generally designed to 160 ~ 200HB, hardness is too low or too high are not conducive to machining operations. Repair welding hardness is too high, will make its plasticity decreased, reducing the valve shell bearing the safety performance. 5, the conclusion steel defects welding science, is an energy-saving remanufacturing engineering. With the help of modern testing methods, innovation and improvement should be continuously made in the welding tools, welding consumables, personnel and workmanship so as to truly realize the integration of manufacturing and maintenance. Search Keyword: More Valve Technical Articles

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