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Common treatment methods valve defect repair welding of steel castings

1 Overview
In the pressure valve in the pipeline industry, cast steel valves due to the flexibility of its economy and the cost of the design, by the extensive use. However, due to various constraints casting process by casting size, wall thickness, climate, raw materials and construction operations, the casting will be blisters, pores, cracks, shrinkage, shrinkage and inclusion, and other casting defects, especially in sand casting alloy steel Castings for more. Because the more alloying elements in steel molten steel flow worse, casting defects on the more susceptible. Therefore, defect identification and formulate a reasonable, economical, practical and reliable valve repair welding repair welding processes to ensure compliance with the quality requirements after the valve has become an issue of common concern to hot and cold processing. This article describes several common welding steel casting defects methods and experience (welding using the old grades that).
2, defect treatment

2.1, defect judgment
In the production practice, some do not allow welding casting defects, such as cracks penetrating, penetrating defect (wear at the end), honeycomb pores, unable to clear the sand slag and area of more than 65c㎡ of shrinkage, as well as both sides the other major flaw can not be agreed in the contract welding. Before welding should determine the type of defect.

2.2 defects removed    Can be used in factories generally blow off carbon arc gouging casting defects, defective parts and then polished using hand-held angle grinder to expose the metallic luster. But production practice more directly with large current to remove defects in carbon steel electrode and grind angle grinder with a metallic luster. Eliminate casting defects generally available <4mm-J422 electrode, 160 ~ 180A current, defect except clean, grinder will polished into a U-shaped mouth defects, reduce welding stresses. Defects cleared completely, good welding quality.

2.3, defective parts preheat
Carbon steel and austenitic stainless steel castings, welding parts of the area where <65c㎡, 20% depth <casting thickness or 25mm, generally do not need to warm up. But ZG15Cr1Mo1V, ZGCr5Mo such as pearlite steel castings, due to the tendency hardened steel large, easy to crack cold welding, preheating should be, preheat temperature is 200 ~ 400 ℃ (with stainless steel electrode welding, temperature take a small value), holding time should not be less than 60min overall preheat as castings, available oxygen - acetylene after the defect site and extend 20mm heated to 300-350 ℃ (dorsal visual observation dusky dark red), large neutral flame torch gun first in the defect and the surrounding circular quick swing a few minutes, then slowly move to keep 10min (depending on the defect thickness), so that defective parts fully preheated, rapid welding.
3, welding method

3.1 Requirements
When welding of austenitic stainless steel castings, to the ventilation, rapid cooling so. Of pearlite welding low alloy steel castings and steel castings area is too large should choose lee or obscured by the windshield to avoid cracks caused by rapid cooling. Welding a heap layer should immediately remove the dregs after welding and hammering out evenly along the defect centers, reduce welding stresses. If the sub-layers of welding performed (usually 3 ~ 4mm for a welding layer), then each should be promptly removed after welding are the dregs and hammering welding area. As in winter welding, ZG15Cr1Mo1V class pearlite alloy steel castings, welding each layer is also applied oxygen - acetylene repeated heating, then quickly welding to avoid weld cracking.

3.2, welding process
Before welding, welding rod should first check whether the warm-up, general welding shall be subject to 150 ~ 250 ℃ drying 1h. Electrode should be set after the warm-up incubator, so used with the check. Welding preheat repeated three times, if the electrode surface coating has come off, cracking and rusting, should not be used.

3.3, welding times
Pressure castings, such as the valve casing pressure test water seepage, the same site generally only allowed once welding, repair welding can not be repeated, because several welding steel coarse grains would affect the performance of the casting pressure, unless castings can after re-weld heat treatment. Other parts of the same non-pressure welding, welding general provisions are not more than three times. The same place more than twice welding carbon steel castings, stress relieved after welding should be treated.

3.4, welding layer height
Welding is generally higher than the height of the casting casting plane about 2mm, to facilitate machining. Welding layer is too low, easy exposed after machining burr. Welding layer is too high, time-consuming cost materials.

4, after the welding process

4.1 Important welding
Hydrostatic test leakage of castings, welding area> 65c㎡ castings, depth> 20% 25mm thickness of the casting castings, ASTMA217 / A217M-2007 or are considered to be important in welding. Welding A217 on such important criteria proposed, should be carried out to deal with stress or fully coupled thermal treatment, and this is completely coupled to deal with stress or heat treatment, must be qualified by the validation method, namely the need to develop important welding welding processes. ASTMA352 / A352M2006 stipulates that after welding important to stress or post-weld heat treatment is mandatory. And A217 / A217M our industry standard corresponding JB / T5263-2005 important welding will be defined as "heavy defects." But in fact, in addition to rough castings can be fully coupled with the heat treatment, many defects are often found in the finishing process, and has been unable to fully heat-treated. Therefore, the production practice, usually by experienced welders welded pressure vessel holds a certificate in the field with an effective way to solve.

4.2, eliminate stress
After finishing welding defects found, has been unable to do the whole stress relieving and tempering treatment, generally can be defective parts oxygen - acetylene flame local heating tempering methods. Using large neutral flame torch swinging slowly back and forth, the casting is heated to the surface visually visible dark red (about 740 ℃), insulation (2min / mm, but not less than 30min). After eliminating stress treatment should be immediately covered with asbestos in the defect. Defective valve size on pearlite steel, welding also when filling in the lumen diameter asbestos, making slow cooling. Such operations, both simple and economical, but requires welders have some practical experience.
After welding stainless steel castings for treatment generally do not, but should be in the ventilation welding, welding area so fast cooling. Show unless welding has caused changes in austenite, or belonging to heavy defects. In the contract and conditions permit, should redo solutionizing. Defect area is too large too deep pearlite steel castings and various castings, castings in clearing stage and into the rough, though, but left the finishing allowance shall be implemented in stress relieved after welding. Carbon eliminate stress tempering temperature can be set 600 ~ 650 ℃, ZG15Cr1Mo1V and ZGCr5Mo tempering temperature can be set to 700 ~ 740 ℃, ZG35CrMo tempering temperature to 500 ~ 550 ℃. All kinds of steel castings, which eliminates stress tempering holding time was less than 120min, and with the furnace cooled to below 100 ℃ baked.

4.3, NDT
For "heavy flawed" and "important welding" valve castings, ASTMA217A217M-2007 standard requirements, such as casting production in line with the provisions of S4 (magnetic particle inspection) additional requirements, welding inspection of castings to adopt the same standards of quality magnetic particle inspection to check . Such as casting production in line with the provisions of S5 (radiographic examination) additional requirements for leaking hydraulic test castings, welding or prepare any pit deeper than the wall thickness of 20% or 1in1 (25mm) casting and welding preparation any pit area of about more than 10in2 (65c㎡) welding castings, castings should adopt the same standards ray inspection inspection inspection. JB / T5263-2005 standard provisions should be re-ray or ultrasound to detect defects after welding. That is important for heavy welding defects and must be an effective non-destructive inspection, before using proven qualified.

4.4 rating
Non-destructive inspection area for welding defects reported grade, JB / T3595-2002 stipulates groove for steel casting and welding parts valves valve power plant shall be GB / T5677-1985 be assessed, three qualified. Valve butt welds shall GB / T3323-1987 be assessed, two qualified. When JB / T644-2008 in the presence of casting defects simultaneously two different grades also gives clear that the presence of two or more types and grades of different defects in the same time zone assessment, whichever is the lowest level as a comprehensive rating . The simultaneous existence of two or more types, and the same level of defects, its comprehensive grade level should be reduced.
For the welding area defects slag, not fusion and lack of penetration, JB / T6440-2008 stipulates that can be seen as casting defects assessed slag, porosity defects in welding zone can be seen as casting defects pores assessment .
The general condition of the valve in order contract is not marked valve castings grade, less marked after passing grade welding defects in the contract, which often give the valve production, testing and marketing brings many contradictions. According to our current level of actual quality steel castings and years of experience, the level of assessment welding area should not be less than is generally believed that the GB / T5677-1985 three, namely ASMEE446b standards for grade Ⅲ. Acid corrosion of steel pipeline operating conditions and high pressure cast steel valve casing pressure valve parts, generally should reach ASMEE446bⅡ level or above criteria. Ray examination results showed that the defect area in line with standard procedures and norms welding, the weld defects generated during, even less than the casting itself, higher level. In short, the welding as part of the manufacturing process, can not be taken lightly.

4.5, hardness testing
Welding zone despite NDT inspection and approval, but For machining, welding should then check the hardness of the area, which is checked for stress relief treatment effect. If the tempering temperature is not enough, or lack of time, can cause high strength weld metal welding area, the plastic is poor, when machining welding area will be very hard, easily lead to the tool to crack. Base metal and weld metal properties inconsistent, but also easily lead to local stress concentration, welding transition obvious traces the emergence of the junction. Therefore, the welding area need to identify and hardness testing. Lightly polished with a hand grinder welding area, using a portable Brinell hardness hammering three points, the welding zone hardness and hardness of steel castings own value. If the hardness value of the two areas are similar, then the oxygen - acetylene basically successful tempering. If the welding zone hardness value greater than steel castings hardness of 20 or more, we recommend rework until close to the hardness of the base material. Confined by the hardness of the general design of steel castings after heat treatment for 160 ~ 200HB, hardness is too low or too high is not conducive to machining operations. Welding zone hardness is too high, it will make the plastic drop, reducing the valve body bearing security.

5 Conclusion
Scientific welding steel casting defects, is an energy-efficient remanufacturing engineering technology. In coordination with the modern testing methods, should continue to be innovative improvements in the welding tools, welding consumables, personnel and technology, real manufacturing and integration of maintenance.
Above all finishing work according to the problem encountered in practice is made, for reference, any questions, please timely communication, correct me.