1.1 Weldability of chromium molybdenum heat resisting steel
Chromium and molybdenum are the main alloy elements, pearlite heat-resistant steel significantly improve the high temperature strength and high temperature oxidation resistance of metals, but they make metal welding performance, welding seam and heat affected zone in which has a high tendency, quenching in air cooling after welding is easy to produce the hard and brittle martensite structure, not only affects the mechanical properties of welded joint, and produce a lot of stress, Thus cold crack tendency.
Therefore, the main problem of heat resistant steel welding is cracks, and the formation of cracks are three elements: structure, stress and hydrogen content in the weld, so it is particularly important to develop a reasonable welding process.
2.1 groove
Groove processing is usually done by flame or plasma cutting process. When necessary, cutting should also be preheated, and PT inspection should be done after polishing to remove the cracks on the groove. Usually choose v-type groove, groove Angle is 60°, from the point of view of preventing crack, groove Angle is more favorable, but increase the welding amount, and the groove and inside both sides of the ground clean, remove oil, rust and water and other dirt (hydrogen, prevent porosity).
2.2 set on
Request cannot enforce group to prevent to produce internal stress, because of its high chromium molybdenum alloy steel crack tendency, therefore, when the welding seam of the constraint degree not too large, WeChat public number: king of welding, so as not to cause excessive stiffness, especially in thick plate welding, prevent weld free shrinkage of brace, jig and fixture, should avoid to use.
2.3 Selection of welding method
At present, the commonly used welding methods for pipeline welding in petroleum and petrochemical installation units are GTAW for the bottom and electrode arc welding for the cover. Other welding methods include MIG welding, CO2 gas welding, electroslag welding and automatic submerged arc welding, etc.
2.4 Selection of welding materials
The principle of selecting welding materials, the alloy composition and strength performance of weld metal should be basically consistent with the corresponding index of the base metal or should reach the lowest performance index proposed by the technical conditions of the product. And in order to reduce the hydrogen content should be with alkaline low hydrogen type electrode, electrode or flux should be in accordance with the provisions, drying technology, along with it, WeChat public number: micro welder, put them in electrode increases with the increasing use in heat insulation barrels, electrode to no more than 4 hours in the heat insulation barrels, otherwise should resume drying, drying times shall not be more than three times, in the process of concrete construction, have detailed provisions. Austenitic stainless steel electrode, such as A307, can also be used for manual arc welding of chromium-molybdenum heat resistant steel, but it still needs to be preheated before welding. This method is suitable for the situation that the welding parts cannot be heat treated after welding.
2.5 preheating
Preheating is an important technological measure for cold crack and stress elimination in welding pearlite heat-resistant steel. In order to ensure welding quality, preheating and maintaining a certain temperature range should be carried out in both spot fixing and welding process.
2.6 Slow cooling after welding
After welding slow cooling is the principle of welding chromium molybdenum heat resistant steel must be strictly followed, even in the hot summer must do this, generally immediately after welding with asbestos cloth covering the weld and near the seam area, small welding pieces can be placed in asbestos cloth slow cooling.
2.7 Post-welding heat treatment
Heat treatment should be carried out immediately after welding in order to prevent delayed cracking, eliminate stress and improve microstructure.
