1、 Selection of welding materials
In the work of dewaxing precision casting parts, we often hear the word "high material, low use". Many people take this word as the standard for selecting welding materials, especially in the welding of dissimilar steel, the welding materials are often selected by the side with higher material grade. In fact, this is a misunderstanding caused by not carefully reading and studying the welding procedures. It is clearly stipulated in the welding procedure that when dissimilar steel is welded, when both sides of the steel are not austenitic stainless steel, the welding rod and welding wire with composition between the two or matching with the side with low alloy content can be selected; When one of the two sides is stainless steel, stainless steel welding rod and welding wire with high nickel content can be selected. For example, T91 welding material has always been used in our factory when welding dissimilar steel joints of 12Cr1MoV and T91. In fact, according to the regulations and welding procedure qualification made by relevant departments, it can be seen that the performance of welded joints welded by welding material of 12Cr1MoV steel fully meets the needs of engineering, and has the following two advantages: 1. Simplified welding process, Improve work efficiency; 2. The maintenance cost is saved.
For dewaxing precision casting parts, the "high material and low use" should be applied to the selection of boiler steel. For example, when the steel at a certain position of the heating surface of the boiler needs to be replaced, and there is no same steel at that time, the material grade higher than the original steel should be selected for replacement, but the steel grade lower than the original steel cannot be selected for replacement.
2、 Size of welded joint
1. Surplus height:
Weld reinforcement of dewaxing precision casting is the height of weld metal higher than the surface of base metal. In the construction, some people think that increasing the reinforcement of the weld can increase the strength of the weld, so it is often required to weld more layers of the welded joint, which is also a wrong understanding. The increase of weld reinforcement can not increase the strength of the weld, but can enhance the stress concentration of the weld. The reinforcement of welding seam is also specified in the specification. The reinforcement of general welding joint should be controlled within 0 ~ 3mm, and that of important welding joint should be controlled within 0 ~ 2mm.
2. Root clearance and groove angle:
In the welding construction of Guangdong mechanical processing plant, the welding process card clearly stipulates the root gap and groove angle of the weld. However, due to various reasons in the actual construction, the root gap and groove angle are too large or too small. Some people think that the root gap and groove angle do not matter, as long as the welder has good technology and can weld. This is also a wrong view, the weld is the worst performance of the whole structure, therefore, the requirement of weld size is to ensure the welding construction and welding quality, the smaller the weld size, the better. In the 5.12 accident in our factory, part of the reason is that the gap at the root of the weld is too large, resulting in a large number of coating flowing into the root.
3、 Welding current
The welding current is an important parameter in the welding process of Guangdong mechanical processing plant. The welding current of each layer of weld is clearly specified in the process card. However, in the actual welding construction, due to the lack of on-site supervision by welding technicians, some welders choose the welding current according to their own habits. In order to avoid slag inclusion and incomplete penetration of the weld, some welders choose the welding current according to their own habits, Large welding current is usually used. The influence of welding current on welding common steel is not too big, but it has a great influence on welding high alloy steel and stainless steel. When welding high alloy steel, if the welding current is too large, the microstructure of weld metal will be changed, and the high temperature serviceability of welded joint will be seriously reduced, which will lead to premature failure of welded joint and accidents. More importantly, the non-destructive testing methods we use now can not detect the performance degradation of welded joints caused by excessive welding current.
The above problems often occur in the actual welding construction. To improve the quality of welding joints, we must do the following:
1. Strengthen the staff (welder, fitter) on the welding procedures and welding process card learning;
2. The construction shall be carried out in strict accordance with the welding process;
3. Further strengthen the management of welding technology, especially for the welding work of important parts, on-site supervision should be carried out;
4. Welding personnel should get rid of some incorrect working habits. With the continuous development of metal materials, the requirements for welding process are becoming more and more strict, and they can't make empiricist mistakes again;
5. Welding workers should constantly learn new technology and new methods, so that their technical level can keep up with the development of welding technology.