Abstract: Cast iron is usually repaired when it encounters some casting defects. Since the 1970s, cast iron welding technology has been widely used, and welding materials have also formed a series of products. Different welding methods should be selected depending on the material, size, thickness, complexity, type and size of defects, and machining and technical requirements of the casting. The following small series gives you a detailed description of the cast iron welding steps and cast iron welding precautions.
Cast Iron Welding Steps Cast Iron Welding Precautions
Introduction
Cast iron is usually repaired when it encounters some casting defects. Since the 1970s, cast iron welding technology has been widely used, and welding materials have also formed a series of products.
Welding method
Depending on the material, size, thickness, complexity, type and size of the defect, as well as the machining and technical requirements of the casting, different welding methods are selected. And prepare for welding according to different welding requirements, such as removing oil and sand, opening or preheating. Welding methods include gas welding, brazing, manual arc welding, and manual electroslag welding. Among them, gas welding is divided into heat welding method, heating reduction zone method, and non-preheating gas welding; manual arc welding is divided into cold welding, semi-heat welding, non-preheating welding and heat welding.
Welding rod selection
The electrode is generally selected according to the post-weld technical requirements. Gray cast iron non-machining cold welding method can be used for oxidized steel core cast iron welding rod (Chinese brand Z100), iron powder steel core cast iron welding rod (Chinese brand Zll2Fe) and low carbon steel welding rod (Chinese brand J422, J506, etc.); heating above 400 °C For hot welding, steel core graphitized cast iron welding rod (Chinese brand Z208) can be used; if the processing surface is not preheated, cast iron core cast iron welding rod (Chinese brand Z248) can be used; it can be processed, crack resistant but not high strength and can be cold welded. Copper-nickel welding rod (Chinese brand Z508) can be used; copper-iron cast iron welding rod (Chinese grade Z607, Z612) can be used with good crack resistance, poor workability and low strength; important castings can be processed, and pure nickel cast iron welding rod can be used. (Chinese brand Z116, z117); high-strength gray cast iron, ductile iron can be cold-welded, iron-iron cast iron electrode (Chinese brand Z408) and steel 927-core graphite spheroidized cast iron electrode (Chinese brand Z268); ductile iron heating When welding, steel core ductile iron electrode (Chinese brand Z238) can be used; thin-wall casting can be processed by steel core graphite spheroidized universal cast iron electrode (Chinese brand Z268 and Z607, Z612) Z268. There is also a new material CaO-BaO-caF2 slag-based steel core graphitized welding rod, which uses the two phase transformation stress relaxation effects of bainite and martensite to improve the crack resistance. China Z238SnCu electrode has high mechanical properties, small white mouth tendency and good crack resistance. It can be used for ductile iron castings. Welding defects and their prevention of white-mouth welding produce white openings in the weld and fusion zone due to the fast cooling of the weld, the insufficient graphitization of the weld of the homogenous electrode or the impediment to graphitization. The way to prevent white mouth is to enhance the graphitization ability of the homogenized electrode and slow down the cooling rate above 800 °C; according to the wall thickness of different castings, it can be preheated to 400-700 °C to prevent white mouth; nickel base and copper are used. Heterogeneous electrodes such as base and high vanadium steel and other measures or brazing can also prevent the generation of white spots.
Thermal stress cracking When the welding force should exceed the strength of the cast iron, cracking along the weak, fusion or heat affected zone of the weld repair zone will cause the weld to peel off. Preventing such cracks starts with reducing stress:
(1) Using the hot welding method, the casting is preheated to 600-700 °C before welding, and the welding process is maintained at 4 () 0 °C or above.
(2) Adopting the heating reduction zone method, correctly selecting the heating position and method to heat the casting, so that the original crack gap is enlarged, and then welding.
(3) Correctly apply arc cold welding, change the chemical composition and alloy system of the weld to make the weld have better plasticity and lower hardness, and use short weld bead to weld the weld and control the temperature rise of the weld repair zone. And other technological measures.
Hot cracks Hot cracks are always perpendicular to the weld fish scales, with longitudinal, oblique and transverse directions. The main reason for this is the increase in the content of carbon, sulfur, phosphorus and silicon in the weld metal. High-vanadium cast iron welding rods are not easy to generate thermal cracks due to the combination of vanadium and carbon. The weld metal is too long in the low plastic zone at high temperatures, and the narrow and deep molten pools are prone to cause hot cracks. The prevention methods include: firstly, improve the thermal cracking resistance of the welding rod, such as enhancing the alkalinity of the coating, reducing the sulfur and phosphorus content, appropriately adding substances such as rare earth and ferromanganese, and selecting an alkaline low-hydrogen welding rod. Secondly, a smaller current is used to reduce the penetration depth, the weld position is inclined, and the vertical welding and vertical welding are used to speed up the welding speed, and the welding rod does not move laterally, so that the bottom of the groove is rounded, avoiding sharp corners, and when the arc is closed Fill the crater and so on.
Cast iron welding step
1. According to the cast iron welding product drawing and technical conditions, the batch quantity and the required date of the product, the casting method is selected according to the actual conditions of the factory.
2. Draw the machining allowances on the various views of the cast iron and the process symbols such as the non-cast holes and grooves.
3. Cast iron welding plots the position, shape, size and quantity of the gating system, the riser, the shape, position and size of the cast sample.
4. Cast iron welding marks the draft angle of the vertical wall of the parting surface.
5. Mark the cast iron casting position and the parting surface.
6. Draw the shape of the sand core, the core block line (including the negative core), the core gap, the compression ring and the anti-pressure ring, and the sand trap. Mark the relevant dimensions and the core, and design the core if necessary. Shape, size and amount of sand.
7. Analyze the structural and technological properties of the castings, judge the tendency of defects, propose structural improvement suggestions and determine the principle of welding and solidification of cast iron.
8. The negative type of the pattern, the shape of the parting surface and the shape of the block, the size and position of the anti-deformation, the negative margin of the non-machined wall thickness, and the position and size of the process correction amount.
9. The position, shape, size and quantity of cold iron and cast ribs, fixed combination method and cold iron seam size.
Note the length of the arc
The length of the arc is related to the type of electrode coating and the thickness of the coating. However, short arcs should be used as much as possible, especially low hydrogen electrodes. Long arcs can cause pores. The short arc can prevent the harmful gases such as O2 and N2 in the atmosphere from invading the weld metal, forming undesirable impurities such as oxides and affecting the quality of the weld.
Welding speed
The appropriate welding speed has corresponding changes in the conditions such as the diameter of the welding rod, the type of coating, the welding current, the heat capacity of the object to be welded, and the structure, and the standard cannot be specified. Maintaining a suitable welding speed, the slag can cover the molten pool very well. The various impurities and gases in the molten pool are sufficiently floated to avoid the formation of slag inclusions and pores in the weld. If the speed of the rod is too fast during welding, the shrinkage stress will increase when the welded part cools, causing cracks in the weld.
The main points of welding wire selection
The selection of the welding wire should be based on the type of steel to be welded, the quality requirements of the welded parts, the welding conditions (thickness, groove shape, welding position, welding conditions, post-weld heat treatment and welding operation waiting), and cost.
The order in which the wire is selected for consideration is as follows:
1Selecting the welding wire according to the steel type of the welded structure For the carbon steel and the low-alloy high-strength steel, the welding wire satisfying the mechanical properties is selected according to the principle of "equal strength matching". For heat-resistant steel and weathering steel, the main consideration is to consider the similarity between the weld metal and the chemical composition of the base metal to meet the requirements of heat resistance and corrosion resistance.
2 According to the quality requirements of the welded parts (especially impact toughness), the welding wire is selected according to the welding conditions, the shape of the groove, the mixing ratio of the shielding gas, etc., and the maximum welding efficiency and reduction are selected under the premise of ensuring the performance of the welded joint. Welding materials for welding costs.
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