Preliminary study on steel barrel welding process (3)

Preliminary study on steel barrel welding process (3)

Sinopec Sales Company North China Company Tianjin Co., Ltd. Sun Lianhe

5. Solderability of metal materials

While discussing the steel barrel welding process, the study of the weldability of metal materials should not be neglected, which is one of the basic conditions for metal welding.

1. The concept of solderability. The solderability of metallic materials can be discussed in two ways.

(1) Principle Solderability: Refers to the ability of metals to crystallize each other during welding to form a permanent joint by intragranular bonding. In general, the solderability of the same kind of metal is better.

(2) Process weldability: refers to the ability of metal materials to obtain high quality welded joints under certain welding process conditions. The weldability we want to study is the process weldability.

The weldability of metal materials is mainly determined by the chemical composition of the materials, but it is closely related to the complexity of the structure, the rigidity, the welding method, the welding materials and the welding process conditions.

2. The weldability of steel. Most of the metal barrels we use today are steel drums. Due to the different chemical composition of the steel, the weldability is also different. Among the various chemical elements of steel, carbon has the greatest influence on solderability. We often use the amount of carbon in steel as the main indicator for judging the weldability of steel. The higher the carbon content of the steel, the worse the weldability of the steel, so the materials used in the steel drums are basically low carbon steel.

In the low carbon steel we use, in addition to the influence of carbon on the weldability, other elements such as manganese, chromium, nickel, copper, molybdenum, etc. have certain influence on the weldability. We classify these elements into comparable carbon content, ie carbon equivalent, based on their effect on the weldability. Generally, the formula for calculating the carbon equivalent CE we use is:

This formula applies to the following components of carbon steel and ordinary low alloy steel: C <0.6%; Mn <1.6%; Ni <3.3%; Cr

According to the calculation, we believe that the general CE <0.25%, its solderability is good; when CE = 0.25 ~ 0.35%, its solderability is general; CE = O. When the temperature is 35 to 0.45%, the weldability is poor; when CE>0.45%, the weldability is not good.

In the production of steel drums, our various steel drum manufacturers have used different types of low-carbon steel in many countries, and their weldability is also different. While we introduced the US welding machine, we used the Beijing Co., Ltd. to commission the US Caroline Machinery Plant to use the radiometric spectrometer to conduct chemical analysis and carbon equivalent calculations for four types of steel in three countries. The results of the analysis are listed in Table 4 for your reference.

Table 4 (unit: %)

Foreign experience suggests that the carbon equivalent for high-speed resistance-welded steel is 0.09% or lower. China's hot-rolled steel sheet has a carbon equivalent of 0.22%. Although it is in the range of good weldability, even if the surface is sufficiently cleaned, it is still a hard-to-weld steel plate on the welding machine provided abroad. In addition to its high carbon equivalent value, we did see from the above table that its silicon content is also very high. It is well known that the content of silicon concentrates to reduce the weldability of steel. At the same time, the steel also produces pinholes, severe splashes and welds on the weld. Therefore, in order to ensure its good solderability, we must take certain technological measures to ensure good welding quality.

Sixth, welding process specifications and their parameter selection

In the production of steel drums, the welding quality of spot welding, seam welding and projection welding has a great relationship with the performance of the welding machine and the welding procedure specification. Therefore, specific factors affecting the quality of the welding process and equipment should be considered in order to select equipment for different materials, structures and use requirements, and to formulate process specifications.

The so-called welding procedure specification refers to the relevant parameters that determine the characteristics of the welding specification that constitute the whole process of welding. There are mainly welding current I1 welding pressure F, welding passing time t, electrode working end face geometry and size. There is a close relationship between these parameters. They can all be varied within a fairly large specification so that the parameters can be reasonably selected and a good weld is obtained. In order to correctly select the parameters of the welding range, the characteristics, functions and mutual interference relations should be mastered.

1. The relationship between the parameters in the specification. The parameters of the welding are mutually restricted and mutually influential. Changing any of these parameters will imply other parameters. Changing the I, F, t, a (b) in the parameters will affect the heat generated at the weld. F, a (b) directly affect heat dissipation, and t and F are closely related to the size of the plastic zone. Increase I and t, lower F to increase the core size, and vice versa. According to the information, when two or more parameters reach the limit of their allowable fluctuations at the same time, the welding quality may be lowered. For example, when I or t is reduced by 5%, and F is increased by 10% or the electrode radius is more than doubled, the amount of heat is lowered and the cooling is strengthened, and the phenomenon of incomplete penetration occurs. When I or t increases by 5%, and F decreases by 10% or R decreases, the influence of each parameter is similar due to the increase of precipitation heat, poor heat dissipation, and occurrence of defects such as splashing, overheating, and deep crater in individual solder joints. Where. However, it should be noted that although several parameters can enhance the heat generation, the heating parameters of the different parameters are different when heated. Therefore, the relevant parameters can be adjusted under the condition that the heat is substantially the same, and the heating speed can be changed to adapt to the requirements of different materials, so when selecting a certain parameter, other parameters must be taken into consideration.

2. Factors to be considered in the selection of welding specifications. The choice of welding specifications and the properties of the weldment material, especially related to physical properties such as electrical conductivity, thermal conductivity, high temperature strength, melting point, plastic temperature range, deformation resistance, hardness, sensitivity to thermal cycling and performance during heating The characteristics of change are related. Among them, the most influential ones are conductivity, thermal conductivity and high temperature strength. In the selection of welding specifications, it is usually the conductivity of the reference material, the current I is selected, the thermal conductivity and the high temperature force F are selected, and then corrected according to other properties (sensitivity to cracks or other defects). For example, stainless steel 1Cr18Ni9Ti has a five-fold higher resistivity than low-carbon steel at room temperature (approximate at high temperatures), so the welding current is lower. Due to its high coefficient of thermal expansion, in order to reduce deformation, the energization time should not be too long, and attention should be paid to the welding sequence. Since the yield limit is slightly higher (50-80 N/mm2, low carbon steel is 40-60 N/mm2), the electrode pressure should be slightly larger.

At present, in actual welding, the parameters of the seam welding and spot welding process specifications are generally determined by experimental methods, and the research on determining the specification parameters by calculation method is still not sufficient.

Seven, metal barrel welding process specifications and parameter selection

At present, most of the materials used in domestic production of metal drums are low-carbon steel sheets, and only a small part of them are coated with zinc. Considering the continuous development of metal barrel production, it is possible to use aluminum alloy sheets or stainless steel sheets in the future. Therefore, the welding process specification of low carbon steel is mainly based on the welding of galvanized sheet, aluminum alloy sheet and stainless steel sheet. The selection of welding procedure specifications is preliminarily discussed, and some relevant parameters are listed for reference only.

1, welding of low carbon steel. The low carbon steel has a resistivity 8-10 times larger than that of copper, and the plastic temperature region is wide, which is easy to obtain plastic deformation and does not require high electrode pressure. Since the crystallization temperature region is narrow, the high temperature plasticity is good, and the coefficient of linear expansion is not high, so the hot crack tends to be small. Carbon and trace elements are low, no high melting point oxides, generally no quenching structure or inclusions, so no complicated process measures are required in welding, and good welding quality can be obtained. When a cold-rolled low-carbon steel sheet is used and welded by a soft gauge, the heat-affected zone around the nugget is enlarged due to long-time heating, and the grain growth and softening zone are conspicuous. Therefore, when the strength of the joint is high, it is not advisable to use excessively long pulse time heating. The surface of the hot rolled low carbon steel has a thick oxide layer. If the cleaning is poor, the electrode may be adhered to the surface of the plate. In the heating and expansion of the material, defects such as deep craters may be formed, which seriously affect the welding quality and the life of the electrode, so the scale should be carefully cleaned. The anti-rust oil on the surface of the cold-rolled steel can be extruded outside the weld, and generally does not affect the quality of the weld, and may not be cleaned. When the oil is too thick during welding, it should also be wiped clean so as not to enter the welding zone or increase the electrode loss due to impurities in the oil. Especially in the seam welding, the necessary surface cleaning should be carried out, otherwise there will be pores or cracks, which will affect the steel. The airtightness of the barrel straight seam. Refer to Table 5 and Table 6 for the selection of low carbon steel point and seam welding process specifications.

Table 5 Low carbon copper spot welding specifications (for reference)

Table 6 Low carbon steel seam welding specifications (for reference)

The energization time t in the table is proportional to the plate thickness δ, and the current density I is inversely proportional to the plate thickness δ. The electrode pressure F should be selected with reference to the welding current I to a suitable value, so that the F which is about to produce a splash is the optimum value. If F is too large, the size of the molten core is small, and thus the expanded plastic ring participates in the force, and as a result, the strength is lowered and the volatility is increased.

2. Welding of galvanized sheets. In order to improve the corrosion resistance of the steel drum, it can be made of galvanized sheet. In the processing, if the galvanized sheet is ground by a grinding wheel before welding or the galvanized layer of the welded edging surface is cleaned by other methods, the selection of the welding procedure specification is basically the same as that of the low carbon steel. However, the weld and the surrounding coating will be severely damaged, reducing the original corrosion resistance. If the galvanized sheet is directly welded without pre-weld treatment, since the melting point of zinc is lower than the melting point of the low-carbon steel, the zinc layer on the contact surface between the plates is first melted during welding, and is squeezed under welding by electrode pressure. The outer edge of the contact surface. It is not difficult to spot weld. In the specification of spot welding of galvanized sheet, the current I should be appropriately increased, and the lower the melting point of the coating, the larger I should be. At the same time, the electrode pressure should be increased by 20 to 25% compared to the low carbon steel spot welding. Refer to Table 7 for the selection of other parameters of the specification.

Table 7 Galvanized sheet spot welding specifications (for reference)

Galvanized sheet seam welding is similar to spot welding. Because the coating has a wider melting range and a severe shunt, a larger current is required to weld. Porosity and cracks are likely to occur during seam welding of galvanized sheets. Tests have shown that the smaller the penetration rate (10-20%), the less crack defects. When the welding speed is high, the surface is overheated, the penetration depth is large, and cracks are likely to occur. Generally, under the condition that the melting and shunting of the coating is increased, a small current is selected as much as possible, and the welding speed is also relatively low, and a strong external water cooling is adopted. At the same time, it should be noted that the electrode surface is often cleaned in time to obtain a satisfactory weld. Refer to Table 8 for specific specifications.

Table 8 Specification for seam welding of galvanized sheet (for reference)

3. Welding of stainless steel and aluminum alloy plates.

(1) Compared with the welding of low carbon steel plates, the aluminum alloy has the characteristics of good electrical conductivity, good thermal conductivity, large linear expansion coefficient, low temperature strength, narrow plastic temperature range and easy oxidation of the surface. Therefore, there are many welding problems in aluminum alloys, and corresponding measures should be taken. For aluminum alloy spot welding, short time, current, and stepped pressure gauges should be used. During seam welding, the current should be further increased (15 to 25% higher than spot welding, and the electrode pressure should be increased by 5-10%. At the same time, it should be noted that the working surface of the electrode wheel is often trimmed. Refer to Table 9 for specific process specifications.

Table 9 Specification for seam welding of aluminum alloy (for reference)

(2) Welding of stainless steel sheets. Stainless steel sheet has a high electrical resistivity (5-6 times that of low carbon steel) and low thermal conductivity (only one-third of low carbon steel), so there is not much difficulty in spot welding and seam welding. A small current, a large electrode, and a slightly shorter heating time can be used. For details, please refer to Table 10.

Table 10 Stainless steel seam welding specifications (for reference)

Above we only discuss the seven problems of the welding process in steel drum production. The relevant theories and process specifications of resistance welding are far from clear. I hope that I can make further exchanges in the steel barrel industry with my own shallow discussion. One of the key processing processes in our steel drum industry, the welding process, has been improved more quickly to promote the further development of steel drum technology.

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