Valve sealing surface material valve welding defects how to deal with

Valve sealing surface material valve welding defects how to deal with

Valve sealing surface is the key working face of the valve, sealing surface quality is related to the service life of the valve, usually sealing surface material to consider corrosion resistance, abrasion resistance, erosion resistance, oxidation resistance and other factors. Usually divided into two categories: soft materials, hard sealing materials.
Valve sealing surface is the key working face of the valve, sealing surface quality is related to the service life of the valve, usually sealing surface material to consider corrosion resistance, abrasion resistance, erosion resistance, oxidation resistance and other factors.
There are usually two main categories:
(1) Soft material
1, rubber (including butadiene rubber, fluorine rubber, etc.)
2, plastic (PTFE, nylon, etc.)
(2) hard sealing materials
1, copper alloy (for low pressure valve)
2, chromium stainless steel (for ordinary high pressure valve)
3, Sitai alloy (for high temperature and high pressure valves and strong corrosion valves)
4. Nickel base alloy (for corrosive media)
Valve sealing surface material selection table
Valve sealing surface material using temperature /℃ hardness applicable medium bronze valve sealing surface -273~232 water, sea water, air, oxygen, saturated steam 316L valve sealing surface -268~31614HRC steam, water, oil, gas, liquefied gas, and other slight corrosion and no erosion of the medium 17-4PH valve sealing surface -40 ~ 40040 ~ 45HRC with slightly corrosive but corrosive media Cr13 valve sealing face -101~40037 ~ 42HRC with slightly corrosive but corrosive media Stalli alloy valve sealing face -268~65040 ~ 45HRC (room temperature)
38HRC (650 ° C) with corrosive and corrosive media Monel alloy K S valve sealing surface -240~48227 ~ 35HRC
30 ~ 38HRC alkali, salt, food, acid solution without air, etc. Hasloy C B valve sealing surface 371
53814HRC
23HRC Corrosive mineral acid, sulfuric acid, phosphoric acid, wet hydrochloric acid gas, chloric acid free solution, strong oxidation medium No. 20 alloy valve sealing surface -45.6~316
-253~427 oxidizing medium and various concentrations of sulfuric acid
How to deal with valve welding defects
1, an overview of the
Among the pressure valves in industrial pipelines, cast steel valves are popular for their economical cost and flexible design. However, due to the constraints of casting size, wall thickness, climate, raw materials and construction operations, casting defects such as trachholes, pores, cracks, shrinkage porosity, shrinkage holes and inclusions can occur, especially in alloy steel castings with sand casting. Because the more alloying elements in the steel, the worse the fluidity of the liquid steel, the casting defects are more likely to occur. Therefore, the defect discrimination and the formulation of reasonable, economic, practical and reliable repair welding process to ensure that the repair welding valve meet the quality requirements has become a common concern of hot and cold valve processing. This paper introduces the repair welding methods and experience of several common defects of steel castings (the electrode is represented by the old brand).
2. Defect treatment
2.1 Defect judgment
In PRODUCTION PRACTICE, SOME CASTING DEFECTS ARE NOT allowed to repair welding, such as penetrating cracks, penetrating defects (through the bottom), honeycomb pores, unable to sand and slag and shrinkage of the area of more than 65 square centimeters, as well as other major defects that cannot be repaired welding agreed in the contract. The type of defect should be determined before repair welding.
2.2. Defect elimination
In factories, carbon arc air gouging can be used to remove casting defects, and then hand-held Angle grinder can be used to polish the defective parts until the metallic luster is exposed. But in the production practice, the defects are removed directly by using the carbon steel electrode with high current and the metal luster is ground by the Angle grinder. Generally, the casting defects can be removed by < 4MM-J422 electrode and 160 ~ 180A current, and the defect mouth can be polished into U shape by the Angle grinder to reduce the welding stress. Defects thoroughly, repair welding quality is good.
2.3. Preheat the defective parts
Carbon steel and austenitic stainless steel castings, where the area of the repair welding part is less than 65cm2, depth is less than 20% or 25mm of the thickness of the casting, generally do not need preheating. However, ZG15Cr1Mo1V, ZGCr5Mo and other pearlitic steel castings should be preheated at a temperature of 200 ~ 400℃(repair welding with stainless steel electrode, the temperature is small) and the holding time should be no less than 60min due to the high hardening tendency of steel and easy cracking of cold welding. Such as casting cannot overall preheating, available oxygen – acetylene in the defect area and expand 20 mm after heated to 300-350 ℃ (micro back visual observation in the dark red), large cutting torch neutral flame gun first rapidly oscillating in the defect and the surrounding is circular for a few minutes, and then to move slowly for 10 min (depending on the thickness of the defect), make the defective parts fully after preheating, fill quickly Welding.
3.2. Electrode treatment
Before repair welding, should first check whether the electrode is preheated, generally the electrode should be 150 ~ 250℃ drying 1H. The preheated electrode should be placed in the insulation box, so that it can be used as needed. The electrode is preheated 3 times repeatedly. If the coating on the electrode surface falls off, cracks and rusts, it should not be used.
3.3. Repair welding times
For confined castings, such as valve shell seepage after pressure test, the same part is generally allowed to be repaired only once, and cannot be repaired repeatedly, because repeated repair welding will make the steel grain coarse, affecting the bearing property of the casting, unless the casting can be heat-treated again after welding. Repair welding of the same part without pressure shall not exceed 3 times. Carbon steel castings repaired for more than two times in the same part shall be treated for stress elimination after welding.
3.4. Repair the height of the welding layer
The repair welding height of the casting is generally about 2mm higher than the casting plane, which is convenient for machining. Repair welding layer is too low, easy to show welding scar after machining. Repair welding layer is too high, time-consuming and laborious material
4, repair welding after treatment
4.1. Important repair welding
In ASTMA217/A217M-2007, castings with leakage during hydraulic test, castings with repair welding area >65cm2, castings with depth > 20% of casting wall thickness or 25mm are considered as important repair welding. It IS SUGGESTED IN A217 STANDARD THAT STRESS REMOVAL OR COMPLETE REHEATING SHOULD BE CARRIED OUT, AND SUCH STRESS REMOVAL OR COMPLETE REHEATING SHOULD BE CARRIED OUT IN A CERTIFIED AND QUALIFIED WAY, THAT IS, REPAIR welding process should BE formulated for important repair welding. According to ASTMA352/A352M2006, stress relief or heat treatment after major repair welding is mandatory. In the corresponding Chinese industry standard JB/T5263-2005 of A217/A217M, important repair welding is defined as “serious defect”. But in fact, in addition to the casting blank can be completely reheated treatment, many defects are often found in the finishing process, can not be completely heat treated. Therefore, in production practice, it is usually solved in an effective way on site by an experienced welder with a pressure vessel welding certificate.
4.2. Eliminate stress
The defects found after finishing repair welding, has been unable to do overall stress elimination tempering treatment, generally can use the defect part oxygen-acetylene flame local heating tempering method. The large cutting torch is used to slowly swing the neutral flame back and forth, and the casting is heated until the surface appears visually dark red (about 740℃), and the casting is kept warm (2min/mm, but not less than 30min). The defects shall be covered with asbestos panels immediately after stress relief treatment. Pearlitic steel valve diameter defects, repair welding should also be filled in the diameter of the asbestos plate, so that the slow cooling. This operation is simple and economical, but requires the welder to have some practical experience.
Stainless steel castings are generally not treated after repair welding, but should be welded in the ventilated place, so that the repair welding area quickly cold. Unless it is indicated that the austenitic structure has been changed after repair welding, or it is a serious defect. As the contract and conditions permit, the solid solution treatment shall be redone. Carbon STEEL CASTINGS WITH LARGE AND DEEP defect areas and various pearLITE castings in the casting cleaning stage and in rough machining, but with finishing allowance, should be treated with stress elimination after repair welding. Carbon steel stress relief tempering temperature can be set as 600 ~ 650℃,ZG15Cr1Mo1V and ZGCr5Mo tempering temperature can be set as 700 ~ 740℃,ZG35CrMo tempering temperature is set as 500 ~ 550℃. For all steel castings, the heat holding time of stress-relieving tempering is not less than 120min, and the castings are released when the furnace cools to below 100℃.
4.3 Nondestructive testing
For “major defects” and “major repair welding” of valve castings,ASTMA217A217M-2007 provides that if the casting production meets the provisions of S4(Magnetic Particle Inspection) Supplementary requirements, the repair welding shall be inspected by magnetic particle inspection of the same quality standard as that of the casting. If the casting is produced in accordance with the supplementary requirements of S5(radiographic inspection), the same injection as the inspection of the casting shall be used for the hydraulic test leakage of the casting, or for the repair welding of any casting whose pit depth exceeds 20% of the wall thickness or 1in1(25mm) and for the repair welding of any casting whose pit area is approximately greater than 10in2(65cm2) Line inspection is carried out. JB/T5263-2005 standard stipulates that ray or ultrasonic inspection should be carried out after repair welding of heavy defects. That is, for heavy defects and important repair welding, must be effective nondestructive inspection, proved qualified before use.
4.4. Grade assessment
As for the grade of non-destructive inspection defect report of repair welding area,JB/T3595-2002 stipulates that the valve groove and repair welding part of cast steel parts of power station valve should be evaluated according to GB/T5677-1985, and the grade is qualified. Valve butt weld shall be evaluated according to GB/T3323-1987, Grade 2 qualified. JB/T644-2008 also gives clear provisions on the existence of two different grades of defects in castings at the same time. When there are two or more kinds of defects with different grades in the evaluation area, the lowest grade is considered as the comprehensive evaluation grade. When there are two or more types of defects with the same grade, the comprehensive grade shall be reduced by one level.
For slag inclusion, non-fusion and non-penetration of defects in repair welding area,JB/T6440-2008 stipulates that the slag inclusion of casting defects can be evaluated, and the porosity of defects in repair welding area can be regarded as the porosity evaluation of casting defects.
The ordering contract of valves under general working conditions does not mark the grade of valve castings, let alone the qualification grade after repair and welding of defects 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 grade of repair welding area assessment should not be lower than the level 3 of GB/T5677-1985, namely the level ⅲ of ASMEE446b standard. The shell bearing parts of cast steel valves and high-pressure cast steel valves under acid-resistant pipeline conditions should generally meet the ASMEE446b ⅱ or above standards. The RESULTS OF RADIOGRAPHIC EXAMINATION SHOW THAT IN THE DEFECT AREA REPAIRED IN ACCORDANCE WITH the standard PROCEDURES AND specifications, the defects generated in the process of cladding are even less and of higher grade than the casting itself. In short, repair welding as part of the manufacturing process should not be taken lightly.
4.5. Hardness test
Although the repair welding area is qualified by nondestructive inspection, but if machining is needed, the hardness of the repair welding area should be checked again, which is also the inspection of the effect of stress elimination. If the tempering temperature is not enough, or time is not enough, it will cause the welding area of the fusion metal strength is high, poor plasticity, machining welding area will be very hard, easy to lead to tool collapse. The properties of base metal and molten metal are not consistent, and it is easy to cause local stress concentration and obvious trace of repair welding transition junction. Therefore, the rewelded area needs to be identified and tested with hardness values. The repair welding area was gently ground with a hand-held grinder, and the three points were hit by a portable Brinell hardness tester. The hardness value of the repair welding area was compared with the hardness value of the cast steel itself. If the hardness values of the two regions are similar, it indicates that oxygen-acetylene tempering is basically successful. If the hardness value of the repair welding area is more than 20 hardness of the cast steel, it is recommended to rework until the hardness is close to the base metal. The hardness of pressure cast steel after heat treatment is generally designed to be 160 ~ 200HB. Too low or too high hardness is not conducive to machining operation. The hardness of the repair welding area is too high, which will make its plasticity decrease and reduce the safety performance of the valve shell bearing capacity.
5, conclusion
The scientific repair welding of steel casting defects is an energy saving remanufacturing engineering technology. With THE HELP OF modern testing methods, continuous innovation and improvement should be made in welding tools, welding materials, personnel and technology, so as to truly realize the integration of manufacturing and maintenance.