Cast steel valves are widely used in pressure valves of industrial pipelines due to their cost-effectiveness and design flexibility. However, due to the constraints of casting size, wall thickness, climate, raw materials and construction operation, various casting defects such as sand holes, blowhole, cracks, shrinkage, shrinkage and inclusions will appear in castings, especially in alloy steel castings cast in sand mold. Because the more alloying elements in steel, the worse the fluidity of molten steel, casting defects are more likely to occur. Therefore, defect identification and formulation of reasonable, economical, practical and reliable repair welding process to ensure that the valve after repair welding meets the quality requirements has become a common concern in cold and hot processing of valves. In this paper, several repair welding methods and experiences for common defects of steel castings are introduced.
2. Defect Treatment
2.1. Defect Judgment
In production practice, some casting defects are not allowed to be repaired, such as penetrating cracks, penetrating defects (bottom penetration), honeycomb blowhole, sand inclusion slag which cannot be removed, shrinkage area over 65cm2, and other major defects that cannot be repaired as agreed in the contract between the two parties. The type of defect should be judged before repair welding.
2.2. Defect Elimination
Carbon arc air gouging is usually used in factories to remove casting defects, and then hand-held angle grinder is used to grind defect parts to reveal metallic luster. However, in production practice, defects are removed directly by high current of carbon steel welding rod and metal luster is grinded by angle mill. Generally, defects in castings can be eliminated by using < 4mm-J422 welding rod and 160-180A current. Defects can be eliminated. Defects can be ground into U shape by angle mill to reduce welding stress. Complete defect removal, repair welding quality is good.
2.3. Preheating of Defect Site
Carbon steel and austenitic stainless steel castings, where the area of the repair welding part is less than 65 cm 2 and the depth is less than 20% or 25 mm of the thickness of the castings, generally do not need preheating. However, pearlite steel castings such as ZG15Cr1Mo1V and ZGCr5Mo should be preheated at 200-400 C (repaired with stainless steel electrode, temperature is small), and the holding time should be no less than 60 minutes, due to the high hardening tendency of the steel and the easy cracking of cold welding. If the casting can't be preheated as a whole, oxygen-acetylene can be used to heat the defect area and extend 20 mm to 300-350 (faint red in the dark by visual observation). The large cutting torch neutral flame gun first swings around the defect area for several minutes, then moves slowly for 10 minutes (depending on the thickness of the defect), so that the defect area can be fully preheated rapid repair welding.
3. Repair welding method
When repairing austenitic stainless steel castings, it is necessary to cool them quickly in the ventilation area. For pearlite low alloy steel castings and carbon steel castings with large repair welding area, the leeward or windshield should be selected to avoid cracks caused by rapid cooling. If a surfacing layer is repaired, the flux slag should be removed immediately after repairing and hammering uniformly along the defect center to reduce the repairing welding stress. If the repair welding is carried out in several layers (generally 3-4 mm is a repair welding layer), the flux slag and hammering repair welding area should be removed in time after each layer of repair welding. For example, in winter welding, ZG15Cr1Mo1V pearlite alloy steel castings are heated repeatedly with oxygen-acetylene at each repair welding layer, and then repaired rapidly to avoid welding cracks.
3.2. Electrode Treatment
Before repair welding, it is necessary to check whether the welding rod is preheated. Generally, the welding rod should be dried for 1 hour at 150-250 C. After preheating, the welding rod should be placed in the insulating box, so that it can be used at will. The electrode is preheated three times. If the coating on the electrode surface falls off, cracks and rusts, it should not be used. 3.3. Repair welding times
Pressure-bearing castings, such as valve shell, are usually allowed to be repaired only once after pressure test and water infiltration. Repeated repairing welding is not allowed, because repeated repairing welding will make the grain in steel coarser and affect the pressure-bearing performance of the castings, unless the castings can be re-heat-treated after welding. For other non-pressurized repairing welding of the same part, it is generally stipulated that the repairing welding shall not exceed three times. For carbon steel castings with repaired welds in the same part more than twice, stress relief treatment should be taken after welding.
3.4. Height of Repair Welding Layer
The repairing welding height of the castings is generally about 2 mm higher than the plane of the castings in order to facilitate machining. The repair welding layer is too low, and the weld scar is easily exposed after machining. Repair welding layer is too high, time-consuming and material-consuming.
4. Repair Welding Post-treatment
4.1. Important Repair Welding
The ASTM A217/A217M-2007 is considered to be an important repair welding for the castings with leakage, repair welding area 65cm2 and depth 20% or 25mm wall thickness. For this important repair welding A217 standard, stress removal treatment or complete reheat treatment should be carried out, and this stress removal treatment or complete reheat treatment must be carried out by a qualified method, that is, important repair welding needs to formulate repair welding process. ASTM A352/A352M2006 stipulates that post-weld stress relief or post-weld heat treatments are mandatory requirements. Corresponding to A217/A217M, China's industry standard JB/T5263-2005 defines important repair welding as heavy defects. But in fact, besides the complete reheating of the casting blank, many defects are often found in the finishing process and cannot be completely heat treated. Therefore, in production practice, it is usually solved by experienced welders with pressure vessel welding certificate in the field with effective methods.
4.2. Elimination of stress
After repairing the defects found after finishing, it is impossible to do the whole stress relief tempering treatment. Oxygen-acetylene flame local heating tempering method can be used in the defect area. The castings were heated to the surface by using large cutting torch neutral flame to swing slowly back and forth. The castings were visually dark red (about 740 C) and kept warm (2 min/mm, but not less than 30 min). After stress relief treatment, asbestos board should be immediately covered at the defect. When repairing the defect in the diameter of pearlite steel valve, asbestos board should be filled in the inner cavity of the diameter to make it cool slowly. This kind of operation is simple and economical, but requires welders to have some practical experience.
Stainless steel castings are usually not treated after repair welding, but they should be welded in the ventilated place to make the repair welding area cool quickly. Unless the austenite structure has been altered or is heavily defective after repair welding. If the contract and conditions permit, the solid solution treatment should be redone. Carbon steel castings and pearlite castings with large defect area and deep defect area are in the cleaning stage of castings and are in rough processing, but they still have finishing margin. Stress relief treatment should be carried out after repair welding. The stress relief tempering temperature of carbon steel can be set to 600-650 C, the tempering temperature of ZG15Cr1Mo1V and ZGCr5Mo can be set to 700-740 C, and the tempering temperature of ZG35CrMo can be set to 500-550 C. For all kinds of steel castings, the holding time of stress relief tempering is not less than 120 minutes, and the castings are cooled to below 100 C with the furnace.
4.3. Nondestructive testing
For the heavy defects and important repair welding of valve castings, ASTMA 217A 217 M-2007 standards stipulate that if the production of castings meets the supplementary requirements of S4 (magnetic powder inspection), the repair welding shall be inspected by magnetic powder inspection of the same quality standard of castings. If the production of castings meets the requirements of S5 supplementary requirements, the same standard of casting inspection shall be used for repair welding of castings leaking from hydraulic test, castings with a pit depth exceeding 20% of the wall thickness or 1 in 1 (25 mm) and castings with a pit area of about 10 in 2 (65 cm 2) to be repaired. Line inspection is carried out. According to JB/T5263-2005 standard, X-ray or ultrasonic testing should be carried out after heavy defect repair welding. That is to say, for heavy defects and important repair welding, effective non-destructive inspection must be carried out to prove that it is qualified before it can be used.
JB/T3595-2002 stipulates that the groove and repair welding position of the cast steel valves of power station valves shall be evaluated according to GB/T5677-1985, and the third level is qualified. Valve butt weld shall be assessed according to GB/T3323-1987, and the second grade is qualified. In JB/T644-2008, there are two kinds of defects with different grades in castings. When there are two or more kinds of defects with different grades in the evaluation area, the lowest grade is chosen as the comprehensive evaluation grade. When there are two or more kinds of defects with the same grade, the comprehensive grade should be reduced by one level.
For slag inclusion, non-dissolving and non-penetrating of defects in repair welding zone, it is stipulated in JB/T6440-2008 that slag inclusion of casting defects can be evaluated, and pore evaluation of defects in repair welding zone can be regarded as pore evaluation of casting defects.
Valve castings are not marked in the order contract of general working conditions, and the qualified grade after defect repair welding is less marked in the contract, which often brings many contradictions to the production, inspection and sales of valves. According to the actual quality level of steel castings in China and many years of experience, it is generally believed that the assessment level of repair welding area should not be lower than the third level in GB/T5677-1985, i.e. the third level stipulated in ASMEE446b standard. Cast steel valves and high pressure cast steel valves under acid and corrosion resistant pipeline conditions should generally meet ASMEE446b class II or above. The results of X-ray examination show that the defects generated during cladding process are less and higher than those of casting itself in the defect areas that meet the standard procedures and specifications for repairing welding. In short, repair welding as part of the manufacturing process should not be taken lightly.
4.5. Hardness Testing
Although the repaired welded area is qualified by non-destructive testing, when machining is needed, the hardness of the repaired welded area should be checked again, which is also a check of the effect of stress relief treatment. If tempering temperature is insufficient or time is insufficient, it will lead to high strength and poor plasticity of the cladding metal in the repair welding area, and the welding area will be very hard during machining, which will easily lead to tool cracking. The inconsistency between the properties of base metal and deposited metal can easily lead to local stress concentration and obvious marks of the transitional boundary of repair welding. Therefore, the hardness value should be used to identify and test the repair welding area. The hardness of the repairing welding area was compared with that of the steel casting itself by using portable Brine hardness tester to hammer three points and grinding the repairing welding area gently with a portable grinding wheel. If the hardness values of the two regions are similar, the oxygen-acetylene tempering is basically successful. If the hardness of repair welding zone is more than 20 hardness of cast steel, it is recommended to rework until the hardness is close to that of base metal. The hardness of pressure-bearing steel castings after heat treatment is generally designed to be 160-200HB. Too low or too high hardness is not conducive to machine work. Too high hardness of repairing weld zone will reduce its plasticity and reduce the safety performance of valve shell bearing capacity.