1 Overview
Among the pressure-bearing valves in industrial pipelines, cast steel valves are widely used because of their cost economy and design flexibility. 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 handling
2.1, defect judgment
In the production practice, some casting defects do not allow repair welding, such as through cracks, penetrating defects (through the bottom), cellular pores, can not remove the sand with slag and more than 65 square centimeters of shrinkage, etc., and Other major defects that can not be repaired by welding agreed in the contract between both parties. Before welding repair should determine the type of defect.
2.2, defect removed
In the factory, carbon arc gouging can be used to cast the defects, and then the angle grinder is used to polish the defects to expose 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. General cast defects removed, available
2.3, the defect site preheating
Carbon steel and austenitic stainless steel castings, where repair welding area
3.2, electrode processing
Repair welding, should first check whether the preheating electrode, the electrode should be generally 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, welding times
Pressure casting, such as the valve shell pressure test seepage water, the same site generally only allow welding repair, welding can not be repeated, because multiple welding will make the steel coarse grains, affecting the casting pressure performance, unless the casting can After welding re-heat treatment. 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 welding height
Casting welding height generally higher than the casting plane about 2mm, 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, welding repair
4.1, important repair welding
Hydraulic tests Castings with seepage, castings with> 65cm2 weld area, castings with 20% or 25mm wall thickness, ASTMA217 / A217M-2007 are considered as important repairs. 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, to eliminate stress
Finished defects found after welding, has been unable to do the overall elimination of stress tempering, the general can use the defective parts of oxygen - acetylene flame local heating method of tempering. 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 "critical repairs" of valve castings, ASTMA217A217M-2007 standard states that if the production of castings meets the requirements of Supplementary Requirements for S4 (Magnetic Particle Inspection), the welding repair shall be inspected by the magnetic particle inspection of the same quality standard of inspection castings . 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, rating
For welding grade nondestructive inspection defect 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
Although the non-destructive testing of welding area inspection qualified, but for machining, 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 Conclusion
Steel welding defects in the scientific, is an energy-saving remanufacturing engineering. With the help of modern testing methods, continuous innovation and improvement should be made in the welding tools, welding consumables, personnel and workmanship so as to truly realize the integration of manufacturing and maintenance.
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