2.Preventive measures

How to prevent welding failure by the formulation of welding procedure, it is the key to analyze and solve the problem. This needs to be considered from the following aspects:

(1) According to the requirements of design and application of structures, such as static load, dynamic load, alternating load, impact load, etc., to choose right materials.

Material selection is not only a technological issue, but also an economic benefit issue. Choosing overhigh quality material is not economic, but selection of  low quality material will reduce the life span and increasing the risk of hidden danger of welding failure.

In the coal mine machinery manufacturing industry, commonly used steel is Q345 (16Mn) and the high strength plate (WH, QJ series )and NM series of wear-resistant plates developed in recent years, In order to ensure welding performance, the content of Ceg in the steel should be <0.45%, S and P content (mass fraction) should not exceed 0.04%.

(2) In order to ensure that the chemical composition and mechanical properties are in accordance with the national standard, the steel products should be strictly checked from purchase to put into production. In addition, the purchase of new varieties of steel,  a small batch test and welding process performance test shall be carried out to determine its technological performance and weldability, and to make a test conclusion, then decide whether or not to be used in large quantities.

The material inspection includes into two kinds: macro and micro. Macroscopic examination is to examine the appearance with naked eyes or magnifying glass. The microscopic examination mainly adopts the methods of nondestructive flaw detection and metallographic section low tissue examination. The aim is to find defects such as crack, interlayer and tear in advance etc, defects. For steel castings, the chemical composition and mechanical properties of the bars should be strictly tested to determine whether they meet the requirements of material properties.

(3) In structural design, the excessive stiffness and stress concentration should be minimized, for example preventing the excessive density of the welding distribution, eliminate the locations where stress concentration may occur. For connection location of components, the location with the greatest stress in the structure should be avoided as far as possible, so as to control the deformation after welding and improve the strength of the components. In addition, according to the use and force condition of its structure, it is necessary to identify the work weld or contact weld, reasonable design of weld distribution, the geometric size of the weld and the height of the weld, at the same time, the economy of welding must be considered. In general circumstances, in order to be strong it is believed that the bigger of welding seam size the stronger the strength of welding for the large size steel structure, in fact, this is a misunderstanding, the correct principle is to meet the basic requirements of weld metal strength, we should minimize the amount of filler metal. It has a close relationship with the structural design and welding process designers for the calculation of structural force and the selection of welding methods and forms. This is a prerequisite for ensuring the excellent welding joint.

Practice has proved that the good design of the joint welding, not only to ensure its strength, but also to reflect the economic value.

(4) Correct selection of welding specification is the key to ensure welding quality and productivity. In welding production, the corresponding welding auxiliary devices are adopted, such as positioning and clamping devices, flip tires, welding positioner and so on. The use of jig can ensure the accuracy of assembly points, facilitate the proper welding position, and facilitate mass production. The training of welders will be strengthened, and the technical level of welders will directly affect the welding quality. The formulation of welding procedure should be fully taken into consideration of the raw material (material), equipment, welding material (filling material), as well as the workers' technical level, the size of production, the environment temperature and so on, and choose the reasonable welding procedure.

In order to avoid hot cracks and cold cracks, pre weld preheating and post weld cooling measures should be taken to avoid hot cracks and cold cracks for the structure with hardened and high strength low alloy steel. In addition, the influence of the temperature on the environment should be considered. In winter, the climate is cold and the material is in the process of cutting and welding, the tendency of crack is increased, and some measures of preheating and slow cooling need to be taken, by means of technology, improve the adverse effect of welding heat cycle so as to ensure good welding joint, only in this way can the hidden trouble of the welded structure be reduced or eliminated.

Finally, the welding joint must be strictly tested, mainly using nondestructive testing methods such as ray flaw detection, ultrasonic flaw detection and magnetic flaw detection so as to find weld defects early and deal with them in time.

3. Summary

Welding failure in steel structure parts often occurs during the use of equipment; the welding failure of any structural part in the equipment will affect the normal operation of the whole equipment and cause incalculable loss to the enterprise.

At present, both at home and abroad are actively exploring and studying the methods to prevent and solve the problems of welding failure, so as to make the welding technology expand in the direction of automation and intelligentization. But for common carbon steel and low alloy steel structure, mastering the rules and improving the existing technology and reducing the occurrence of such problems is also a necessary means for economic growth.

In short, the most important thing is to base on practice, strictly control the quality of steel, and refine the design of welded structure, through the continuous improvement of welding technology innovation, and absorbing advanced welding technology at home and abroad, to prevent and eliminate the consequences of welding joint failure.

Of course, to improve the overall quality of welding engineers is also very important. Therefore, we should seize every opportunity to learn and improve in our work. To this end, all welding engineers can declare the following meeting activities, as well as certificates, which have very strong practical effects.