1 Overview
High fragment steel has been widely used in the weapon industry due to its high mechanical strength, high hardenability and high hardness, and it is dominant in the choice of materials as the main component in weapon design. However, this type of material not only has poor weldability, but is prone to defects, and the welding is usually performed after the material is heat-treated, so that it has higher requirements for satisfying the mechanical properties and welding process of the product.
High fragment steel is mainly composed of high carbon content, many alloying elements and a carbon equivalent of >0.6%. Moreover, the processing flow before welding is annealing→machining→tempering→machining. While maintaining the high hardness and high strength characteristic indexes, it is also necessary to avoid adverse effects on the product performance index after the heat affected zone. For this reason, the welding quality of the welded joints is very high. From the characteristics of various welding methods and the comprehensive factors of production and manufacturing costs, we use argon arc welding.
2. Common defects and causes of steel argon arc welding for high fragmentation
(1) Embrittlement and softening of the heat affected zone of the weld 1 High steel for fragmentation due to high carbon content and many alloying elements, the hardening tendency of steel is large, and the M s point is low, so a large number of hardened horses are produced in the quenching zone. The structure of the body (especially high carbon coarse martensite) causes severe embrittlement. 2 When the steel for the high fragment is steel in the quenched and tempered state before welding, the heat affected zone is heated to a temperature exceeding the tempering temperature of the quenching and tempering treatment, and the softening zone with lower strength and hardness than the base material will appear.
(2) Crack 1 Due to the high carbon content and high alloying elements of the high fragment steel, the supercooled austenite has greater stability, and the hard and brittle martensite structure is prone to the near seam area, which increases the welded joint. The tendency of cold cracking, in order to improve the crack resistance, should reduce the hydrogen content of the welded joint. 2 The steel for high fragmentation has a high content of carbon and alloying elements. When the weld is solidified and crystallized, the crystallization temperature range is large, the segregation tendency is also large, and the welding has a large thermal crack sensitivity. 3 High fragment steel also has stress corrosion cracking sensitivity.
Such stress corrosion cracking often occurs in corrosive media such as water or high humidity air.
(3) Porosity Due to the uncleanness of the base metal joint before welding, the base metal itself has the characteristics of high carbon content, and the new pore is a common defect of the welding of the material.
There are many factors that form pores. Hydrogen is the main factor for the generation of pores. During the cooling process of the weld metal, the solubility of hydrogen changes. For example, when the partial pressure of hydrogen in the atmosphere around the weld zone is high, the hydrogen in the weld metal does not easily diffuse and escape. The pores are formed together.
When the carbon content of the high fragment steel is excessive in the weld, the reaction to form carbon monoxide gas may also cause pores. As the welding current and welding speed increase, as well as the change in the shielding gas flow rate, the pores tend to increase. The purity of argon, the means of protection, and the gas adhering to and adsorbing by external impurities or the change in ambient temperature are also factors that affect the pores generated in the weld.
(4) The inclusions of the tungsten alloy are either detached or the impurities are not cleaned before welding. The defect of steel argon arc welding for high fragmentation is mainly tungsten.
3. Measures to control defects
In view of the problems in the production of argon arc welding, the welding process should be strictly required to control the defects, and the following measures can be taken to ensure the welding quality.
(1) Strictly perform welding process qualification. Strict detection of chemical composition of base metal and filler material, hydrogen and oxygen content in the base metal must be within the technical conditions.
(2) Argon gas having a purity of 99.96% is used as a shielding gas while ensuring a low argon dew point.
(3) Strictly clean the base material and the dirt on the filling material before welding to ensure that the joint of the base metal joint is free of dirt within 30mm. The time from cleaning to welding is no more than 2h.
(4) Avoid the contact between the tungsten electrode and the molten pool, regularly grind the tungsten electrode, strictly implement the welding process, and prevent the overload welding, causing the tungsten to fall off naturally.
(5) Avoid welding in a working environment with low temperature and high humidity.
4. Welding process
(1) Correct selection of welding parameters. Choose a low-matching weld that meets the weld strength of 85% or more of the base metal strength. The filler material is a low-carbon, low-silicon welding wire such as H18CrMoA.
(2) Select a larger heat input as much as possible and extend the residence time of the molten pool to facilitate the escape of gas. Welds below 5mm can be welded once in a single pass with large current; welds with a thickness of 5mm or more should be welded in multiple passes. However, it is guaranteed that the bottom welding is penetrated and the unfused defects cannot occur. In the case of multi-pass welding, the upper limit of the tungsten current is used for the non-filled bottom welding. Ensure that the crater is filled and good weld formation, paying particular attention to the center of the crater of the first layer and the recess of the weld. Before each layer is welded to the next layer, the hammer weld is applied, and the temperature between the weld layers is cooled to 230 ° C and then welded. For example, in the actual production: when the welding base material is φ 10mm and the thickness is 10mm, the bottom welding is made of tungsten pole φ 2.5mm, without wire filling, welding current is 280~300A, ensuring full fusion around the bottom of the hole, thickness 3 ~4mm, then fill and cover welding. Filling wire, welding current 250A ~ 280A, the remaining height of the cover welding is controlled at 0.5 ~ 1mm, the weld is beyond the edge of the hole 1 ~ 2mm, after the bottom welding and cover welding without a round hammer.
(3) Appropriate amount of welding current, resulting in jet transition, such that heat concentration, reduce heat affected zone, increase the transition frequency and speed of the droplet, reduce the partial pressure of hydrogen and the contact time of droplets with air.
(4) Select the appropriate gas flow rate and welding speed. Excessive gas flow rate will increase the cooling rate of the weld, and short-arc welding should be used as much as possible to increase the reliability of the shielding gas. After the flow rate is suitable, the silver-gray should be the best after the weld is cooled after welding.
(5) De-stress tempering (≤250°C, 6～8h) as soon as possible after welding (heat-affected zone temperature ≥100°C).
(6) Try to avoid welding defects and scratches on the surface of the weldment, and reasonably control the residual height of the weld.
5 Conclusion
Production practice shows that argon arc welding can meet the welding of high fragment steel. For the defects of welding, as long as the material composition itself is combined with the characteristics of argon arc welding to take corresponding effective measures, welding defects can be avoided.
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