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“Preheating†is to heat the entire welding part to a suitable temperature before welding, thereby reducing the temperature difference between the welding part and other parts. Low-carbon steels and non-ferrous metals are better in plasticity. They are only welded to large-section parts and are preheated when the temperature is low. The preheating temperature is about 100°C. The preheating temperature of medium carbon steel is 200-250°C. High carbon steel can be preheated to above 300°C depending on the carbon content. Cast iron parts should be preheated to 600 °C.
"Slow cooling" is to try to keep the parts after welding (such as asbestos ash or lime powder), so that the weldment slowly cools to eliminate the internal stress. Slow cooling is particularly important for high-carbon steels, alloy steels, and cast irons. For some of the more demanding weldments, to eliminate the residual internal stress after welding, stress relief annealing can be carried out. That is, the welded parts are slowly heated to a certain temperature in a heating furnace and kept warm for a certain period of time, and then cooled in the air or The furnace slowly cooled. If the cylinder head is hot welded, the following annealing specification shall be applied: Annealing temperature 600°C, holding time for 10 minutes, and then with furnace cooling to below 300°C.
Considering the deformation that may be caused by the self-weight, when the heating temperature is relatively high, the weldment should be supported in advance.
(2) Hammered weld When the surfacing layer or weld seam is in a hot state, tap the weld seam with a small hand hammer to offset the contractile force of the weld metal and the heat affected zone metal, thereby reducing or eliminating the internal stress, Reduce or correct distortion. Hammering the welding site can also change the arrangement of molecules in the metal structure and improve the mechanical properties and corrosion resistance of the metal. The metal with better ductility is more effective with this method. The welds on the bottom and surface layers are generally not hammered.
When hammering, be sure to choose the right temperature range. Some metals are unusually weak at certain temperature ranges, and some metals are brittle. For example, when the temperature rises to 400-500°C, the strength of aluminum is almost lost. When the temperature of the bronze casting rises to 550-650°C, the strength of the bronze casting becomes very small. At this time, a very slight impact or a large static load (eg, dead weight) can cause the weldment to be damaged. Steel materials have blue brittleness at temperatures between 300 and 500°C and cannot be hammered. In addition, steel materials containing high levels of phosphorus are also susceptible to cracking when hammered cold. In general steel materials, the hammering effect is good when the temperature is 800°C. As the temperature decreases, the hammering force should also be reduced. Hammering is not allowed from 300 to 500°C. Hammering of the cold welds should be performed at temperatures below 300°C. When hammering, strike the transverse direction of the weld as much as possible so that the weld metal stretches as far as possible. And the hammer should be dense, light and even.
(3) The longer the weld length is, the longer the heat affected zone is. The more uneven the temperature distribution, the greater the thermal stress and the greater the risk of deformation and cracking of the weldment.
Shortening the length of each weld repair and welding in a reasonable order can reduce the scope of the heat affected zone, reduce the degree of uneven temperature distribution, and make the stress and deformation uniform.
In production practice, weld repairs are generally carried out according to the conditions of the weldments, cracks, and length of cracks, such as subsection back-off method, sub-step inverse symmetry method, and piecewise staggered jump method. For parts that need to be filled with more metal, a multi-layer surfacing can be used. For worn shaft parts, balanced deformation surfacing can be used.
Section 1 back off method. Commonly used for short crack welds. Before welding, the welds are divided into appropriate small segments, marked in order, and back welded. The welding of the weld edge section proceeds from the end of the crack to the center, and the other sections are connected end to end.
2 Sectional inverse symmetry method. This method is mostly used for longer crack welds. Also divide the weld into several symmetrical segments, indicate the sequence, and perform welding repairs. During welding repair, the 1st and 2nd sections are at the center of the crack and are welded to the center from both sides. The rest of the sections are connected by head-to-tail method.
3 section staggered jump method. This kind of welding repair process is basically the same as the above two methods. It is also to divide the weld into many sections and try to weld from the lowest temperature of the base metal of the weld. This kind of welding repair process is mostly used for welding (inlay) filling and pit filling of holes. When welding large holes, the inserts should be made convex.
4 leaf crack repair. When repairing leaf-shaped cracks, first take different measures according to the location of cracks on the weldment and the condition of the branches. In principle, the first short-weld pattern is welded, and the main pattern after the long-weld pattern is: first the crack in the thick part of the welded wall, and then the crack in the thin part of the welded wall. Secondly, it is necessary to select segmented or inter-weld welding according to the length of the crack, and perform welding according to the non-preheating cold welding process.
5 surfacing welding method. "Surface welding" is a multi-layer welding repair. It is often necessary to comprehensively apply the various welding methods mentioned above during surfacing. It is commonly used to weld large weldments. The use of multi-layer welding repair welding surfacing weld cracks should be carried out according to the cold welding operation process. In order to reduce the melting and penetration of the base metal, it is possible to use thin electrodes, small currents, intermittent welding, dispersion welding, etc. in the welding of the first layer, or to braze the first layer; the upper and lower layers The welding direction of the weld should be as opposite as possible. Surfacing, also widely used for surface wear parts, restores weldment thickness, overall dimensions, and strength requirements. When the wear surface of the shaft is surfacing, the balance welding method can be used for surfacing.
(4) Local heating reduction method 1 heating reduction principle. When the metal is heated, it will expand and shrink when it cools. The heating reduction method is to use this rule to control the thermal stress formed during welding repair. Before and after welding, select the appropriate part of the heating, expansion, to offset or reduce the weld stress to prevent cracks. This heated area is called the "reduction area." This method subtly solves the problem of narrowing the temperature difference (refers to the temperature difference between the weld area and those parts of the weldment that hinder free deformation of the weld area). The reduced area after heat welding creates a force that compresses the weld to counteract the residual tensile stress formed during the free shrinkage of the weld zone.
2 heating reduction area selection. The correct selection of the heating reduction zone is the key to this method. One, two or more sites can be selected according to the different locations of the weld. The selection principle is as follows:
Select the site or area that directly affects the expansion and contraction of the weld area, ie heating the area can expand or shrink the weld. Generally, the heat-reducing area before heating and welding should expand the weld. After the heat-welding, the heat-reducing area should shrink the weld.
Reducing areas should be selected as far as possible in the corners of the edge of the ribs and metal areas.
To select areas that do not affect other parts too much, so as not to cause deformation or cracking in other parts.
It must be pointed out that an erroneous heating-reducing zone not only fails to receive the desired effect, but also increases the internal stress.
(5) Pre-adding the reverse deformation method Before welding, according to the experience and the metal properties of the weldment, estimate the size and direction of the deformation, make the weldment deform in the opposite direction in advance, or place the weldment in the opposite position. After due to its own shrinkage deformation, just to achieve the desired normal state.
(6) The rigidity of the rigid fixed weldment is generally small after welding. Therefore, if the rigidity of the weldment is strengthened before welding, deformation after welding can be reduced. For thin-walled parts with a wall thickness of 2 mm or less, and welding of fractured parts, rigid fixation is often required to prevent deformation or misalignment.
There are many ways to fix, some use simple fixtures or supports, some use special fixtures, some are temporary spot welding fixed on the rigid worktable, and some even use the components themselves to form a more rigid assembly (for example, some When the unit welding breaks the crankshaft, the crankshaft is fixed on the body.
Rigid fixing method is very effective for reducing deformation, and it does not need to consider the welding sequence too much when welding. However, for some large and complex parts that are not easy to fix, after the welding is fixed, the weldment will often have a little deformation. In addition, this method will increase the welding stress. We must be cautious when welding brittle materials and prevent cracks.
(7) Forced cooling method Forced cooling method, also known as heat dissipation method, is to quickly transfer the heat of the welding repair site, so that the heat of the base metal near the weld is greatly reduced to achieve the purpose of reducing deformation.
Cold water jet weldment can also be used to achieve the purpose of reducing the temperature of the base metal and preventing deformation.
Some units apply a wet cloth near the weld seam, which can be replaced at any time during welding, and can also play a role in cooling and cooling.
Prevent weldment deformation and cracks
(l) Preheating and slow cooling Preheating and slow cooling can effectively reduce or even eliminate welding stress.