Answer: Temperature field of casting: the temperature distribution line on the cross section of casting at a certain moment of solidification and cooling.
Solidification methods of castings: layer-by-layer solidification, paste solidification and intermediate solidification.
Layer-by-layer solidification: there is no solidification zone, and the solid zone develops layer by layer from the surface to the center.
Paste solidification: the solidification zone is very wide, even running through the whole casting section. The surface temperature seems to be higher than the freezing point TS, and then the surface layer is lower than TS, and the crust begins to form the solidification zone.
Intermediate solidification: the solidification area is wide and quickly extends to the casting center.
2 What is the filling ability and fluidity of the alloy? What is the connection and difference between the two? How to improve the filling ability of alloy?
Answer: The ability of liquid alloy to fill the cavity and obtain castings with complete shape and clear outline is called filling ability.
The fluidity of an alloy refers to the fluidity of the alloy itself.
The internal factors that affect the alloy's fluidity are mainly related to the properties of the alloy itself. Filling ability can be considered as the fluidity of liquid alloy, considering the influence of mold and other technological factors.
In order to improve the filling ability of alloys, alloys with * * * crystal composition or alloys with small crystallization temperature range should be selected as far as possible to improve the solidification quality of molten metal as much as possible. The purer the molten metal, the less gas impurities, and the better the filling ability.
What are the principle of sequential solidification and the principle of simultaneous solidification? What alloys and casting structural conditions are suitable for each?
Answer: The principle of sequential solidification is to adopt various technological measures to gradually solidify each part of the casting from one part to another in the specified direction. This principle is suitable for alloy castings with large shrinkage or large wall thickness difference, such as cast steel and high strength gray cast iron, which are prone to shrinkage cavity.
At the same time, the principle of solidification is to take corresponding technological measures to make the temperature of all parts of cast iron uniform and solidify at the same time. This principle is applicable to gray cast iron with small shrinkage.
4 What are thermal stress and mechanical stress? What is the distribution law of thermal stress and mechanical stress in cast iron? What are residual stress and temporary stress?
A: Thermal stress is caused by uneven cooling speed and linear shrinkage of parts with uneven wall thickness on castings, which hinder each other's shrinkage.
The stress caused by external mechanical obstacles such as mold, core, gate and riser during solid shrinkage of castings is called mechanical stress.
The general distribution law of thermal stress produces tensile stress for thick-walled or slowly cooled parts and compressive stress for thin-walled or rapidly cooled parts.
Mechanical stress is generally tensile stress.
After the casting is cooled to room temperature, thermal stress will remain in different parts of the casting, which is a kind of residual stress.
5 what is the structural manufacturability of castings? What are the requirements for casting structure from the perspective of simplifying casting process?
Answer: The manufacturability of casting refers to the difficulty of casting under the premise of ensuring the quality of casting.
The structure of the casting should make each working procedure in the casting process easy to operate, improve efficiency, reduce cost and ensure quality under the premise of meeting the use requirements, as follows:
1 Shape of casting:
(1) Try to avoid lateral concave on the outer surface;
(2) Make the dividing surface as flat as possible;
(3) Minimize the number of parting surfaces;
(4) The structures such as bosses and ribs should be easy to demould;
(5) The unmachined surface along the demoulding direction should have a structural gradient convenient for demoulding;
2 casting cavity:
(1) Use the kernel as little as possible;
(2) It should be conducive to the fixing and exhaust of the core;
(3) It should be conducive to denuclearizing.
Considering the casting properties of the alloy, what are the requirements for casting structure in order to avoid casting defects?
Answer: the casting structure should also consider the filling ability, shrinkage characteristics, gas absorption and so on. And the influence of casting performance on casting quality to avoid various defects, as follows:
Wall thickness of castings:
1 The casting should have proper wall thickness to avoid casting defects;
2. The wall thickness of the casting should be uniform to avoid defects such as shrinkage cavity, air hole and crack.
The thickness of the inner wall of the casting should be slightly less than that of the outer wall, so that the whole casting can be cooled evenly.
4 wall thickness distribution should conform to the principle of sequential solidification.
Connection of two walls
1 The joint of the wall should have structural fillet, and the fillet size should be suitable for the wall thickness to avoid hot joint.
When the two walls intersect obliquely, acute joints should be avoided, but right-angle joints should be adopted.
3 Try to use staggered joints (central parts) and circular joints (large parts) to avoid cross joints.
4 when connecting walls with different thicknesses, they should be gradually transitioned to avoid sudden changes and easily form stress concentration and cracks.
Structure for avoiding deformation and cracks
The slender flexible casting of 1 should be designed with symmetrical sections, because the symmetrical sections cancel each other, the deformation is greatly reduced.
2. Reasonable setting of reinforcing ribs can improve the rigidity of flat castings and prevent warping deformation.
The wheel width of larger pulleys, flywheels and gears can be made into curved odd-numbered or perforated spokes to reduce casting stress and prevent spokes (spokes) or rims from being slightly deformed and cracked.
What are the mechanical properties, casting properties and other properties of gray cast iron? Which castings are most suitable for gray cast iron?
A: The strength performance is poor, the tensile strength is very low, the plasticity is close to zero, the hardness is extremely low, the tensile strength is much lower than that of carbon steel, and there is almost no toughness, but graphite has almost no harmful effect on the compressive stress of the base part, so the compressive strength and hardness of gray cast iron are similar to those of the base part.
Good castability and machinability, low shrinkage of gray cast iron, chemical composition of gray cast iron close to crystallizing point, and good fluidity.
Good antifriction, good shock absorption and notch sensitivity.
Gray cast iron is widely used to manufacture bearing castings with complex shapes that need to absorb vibration under friction conditions, such as machine tool bed, guide rail, frame, cylinder head and so on.
Why is the strength and plasticity of nodular cast iron higher than that of gray cast iron, but the casting performance is worse than that of gray cast iron? How is nodular cast iron made?
Answer: The normal structure of nodular cast iron is thin and round graphite balls and metal base parts. Because the utilization rate of graphite can reach 70%-90%, the mechanical properties of nodular cast iron are much higher than those of gray cast iron, the tensile strength can even be comparable to that of steel, and the plasticity and toughness are greatly improved, but the fluidity is not as good as that of gray cast iron, so the casting performance is worse than that of gray cast iron.
Manufacture of nodular cast iron;
The chemical composition is strict, and the carbon and silicon content of raw molten iron is higher than that of gray cast iron, so as to reduce the content of manganese, phosphorus and sulfur in nodular cast iron.
The tapping temperature of molten iron is higher than that of gray cast iron, which makes up for the loss of molten iron temperature during spheroidization and inoculation.
Thirdly, spheroidizing treatment is carried out, that is, adding spheroidizing agent into molten iron.
(4) carrying out inoculation treatment.
Five ductile iron has poor fluidity and large shrinkage, which requires higher pouring temperature and larger gating system size, uses more risers and chills, and adopts the principle of sequential solidification.
(6) carrying out heat treatment
9 What is the castability of cast steel? What are the main characteristics of casting technology? What kind of heat treatment is needed for steel castings after casting and why?
Answer: 1 poor castability of cast steel: molten steel has high melting point, poor fluidity, large shrinkage and is easy to be oxidized. Therefore, the casting of cast steel is difficult, and defects such as shrinkage cavity, porosity, residual stress and cracks often appear. Cast steel has coarse grains and lower toughness than forged steel with the same composition, so more complicated technological measures should be taken than cast iron when casting.
2 Process characteristics: (1) requires high strength, fire resistance and air permeability of molding sand.
(2) The principle of sequential solidification is mostly adopted in casting process, and riser and chill are widely used.
(3) casting requires heat treatment
In order to refine grains, improve as-cast structure, eliminate casting stress and improve mechanical properties, steel castings are generally annealed and normalized after casting.
10 What are cold deformation and hot deformation, and what are their characteristics?
A: According to the difference of deformation temperature and deformation structure, the deformation below recrystallization temperature is usually called cold deformation, and the deformation above recrystallization temperature is called hot deformation. Cold-deformed metals show work hardening, and the work hardening of hot-deformed metals is immediately eliminated by recrystallization.
1 1 What is the effect of cold deformation strengthening on the microstructure and properties of metals, and how to make use of its favorable factors in production?
A: When metal is cold deformed, with the increase of deformation degree, strength and hardness increase, and plasticity and toughness decrease. This phenomenon is called cold deformation strengthening, also known as work hardening or cold hardening. During cold deformation strengthening, the crystal lattice near the symmetry plane of the metal is distorted, and even the grain breaks. The strength and hardness of metals are getting higher and higher, while the plasticity and toughness are getting lower and lower. Cold deformation strengthening is one of the means to strengthen metal materials, especially some metals that cannot be strengthened by heat treatment.
What is the effect of 12 recrystallization on the microstructure and properties of metals?
A: If it is deformed metal; When heated to a higher temperature, atoms will have stronger diffusion ability, so that they can grow new equifine grains with the same lattice structure as before deformation, which is called recrystallization. Recrystallization can completely eliminate the hardening phenomenon caused by plastic deformation, make grains harden, and the mechanical properties are even better than before plastic deformation.
What effect does the existence of 13 forging streamline have on the mechanical properties of metals? How to consider forging streamline in the design of mechanical parts.
A: During plastic deformation, the metal grains are stretched or flattened in the same direction. After deformation, the intergranular impurities are also arranged along the deformation direction. This grain boundary impurity distributed along a certain direction is called forging streamline, which makes the mechanical properties of metals anisotropic, that is, the mechanical properties in different directions are different. In mechanical parts, attention should be paid to:
(1) Streamline is consistent with the direction of the maximum tensile stress of the workpiece.
(2) Streamlines are perpendicular to shear stress and impact force.
(3) continuous distribution along the outer contour of the workpiece
14 how to measure the plastic workability of metals, and what are the measures to improve the plastic workability of metals in production?
A: The plasticity and deformation resistance of metal are the comprehensive measures of the plastic workability. Measures to improve the plastic workability of metals in production are as follows:
(1) Because the plastic deformation performance of pure metal is better than its alloy, the forgings should try to choose pure metal or metal alloy with less impurities.
(2) The plastic formability of single-phase structure is better than that of multiphase structure, and it is best to keep single-phase impure solid solution state when forging metal.
(3) In a certain temperature range, increase the temperature of forgings.
(4) Compressive stress state can improve the plasticity of metal. During extrusion, there is compressive stress in the deformed metal, so try to use extrusion processing to process forgings.
(5) Increase the strain rate to exceed the critical rate.
15 What are the similarities and differences between punching and blanking? How to determine the size of punch and die?
A: The similarity between punching and blanking is that the blank deformation and separation process and die structure of the two processes are the same.
The difference between the two: punching is to punch holes in the plate to get parts with holes, and all punching is waste;
Blanking is to obtain a blank with a certain shape and size. The punched part is the finished product, and the rest is leftovers or waste.
Determine the die size: the diameter of the blanking part is the same as that of the female die, and the diameter of the punching hole on the plate is the same as that of the male die. The difference between the two diameters is the double-sided gap value z of the die, that is, D concave =D convex+Z; For stamping die, D convex =D hole, D concave =D convex+Z; For blanking, d concave =D concave-z.
16 what is the deformation process of sheet metal during bending? What defects or waste products are easy to produce and how to prevent them?
Bending deformation process: after the punch descends and contacts with the sheet metal, the sheet metal begins to bend with a large bending radius. The width of the bending part decreases from ~ with the press of the punch, the contact distance between the outer side of the sheet and the working face of the female die decreases from ~, the punch continues to press and decrease, the inner side of the sheet starts to contact with the working face of the male die, then the part of the sheet between the contact point of the punch and the working face of the female die bends in the opposite direction, and the bending radius continues to decrease. Finally, the number of sheets continues to decrease.
Deformation is stressed, the inner metal of the sheet is compressed under the tangential compressive stress, and the outer metal is prone to tensile deformation under the tangential tensile stress. In order to prevent tensile cracking, materials with high strength and good plasticity should be selected as far as possible, and the bending radius should be limited so that the actual bending radius R is greater than the minimum bending radius. During blanking, attention should be paid to make the tangent direction of the curved arc consistent with the streamline direction of sheet rolling, so as to prevent the surface of the sheet from being scratched, so as to avoid the stress concentration caused by the scratched part being in the stretching position.
The measures to prevent springback are: (1) changing the die size and bending angle to make it reach the critical size after springback; (2) Changing the die structure and adopting plastic bending; (3) Using stretch bending technology.
17 why does the springback occur when the sheet metal is bent? What are the factors that affect the rebound value? How to control the springback phenomenon and ensure the bending accuracy?
Answer: After removing the external load, the plastic deformation will remain, while the elastic deformation will recover, which will cause the sheet metal to deform in the direction opposite to the bending direction, resulting in the elastic recovery phenomenon.
The factors that affect the elastic complex value are: mechanical properties of materials, bending radius, bending angle and so on.
The measures to control the springback phenomenon are: (1) changing the die size and bending angle to make it reach the critical size after springback; (2) Changing the die structure and reducing the contact area between the punch and the sheet by plastic bending; (3) Using stretch bending technology.
What are the harmful elements in the weld seam of 18 steel? What are the disadvantages? Where do these harmful elements come from and how to control them?
Answer: The harmful elements in the weld are hydrogen, oxygen, nitrogen, sulfur and phosphorus. These harmful elements will make the properties of weld metal brittle, hydrogen, sulfur and bowls will cause cracks, and hydrogen and nitrogen will also cause pores. Oxygen and nitrogen in the air, moisture in covered electrode coating, flux, crystal water and oil stain in rust will be decomposed into nitrogen, hydrogen and oxygen under the action of high temperature of electric arc and enter the molten pool liquid metal. When nitrogen and hydrogen are dissolved in liquid metal, oxygen and metal will undergo oxidation reaction to form oxides, and alloy elements will burn. Rust, covered electrode skin and submerged arc welding flux will also bring in some oxides. In addition, the parent metal and welding materials will also bring in impurities such as sulfur and phosphorus. The control method is as follows: before welding, remove the rust, water and oil stain on the parts to be welded, dry the covered electrode and flux according to the specified parameters, and take measures to protect the molten drops, liquid metal in the molten pool and high-temperature weld metal during welding to prevent air from entering. Ferroalloy is added to covered electrode coating and submerged arc welding flux, metal elements are added to welding wire for alloying, and elements are added to weld metal to obtain ideal chemical composition of weld metal, thus ensuring the necessary properties of weld metal.
19 What are the areas of the welded joint? Where are the weak areas with poor mechanical properties? Why?
Answer: (1) Welded joints include weld, fusion zone and welding heat affected zone.
(2) The chemical composition of the fusion zone is uneven, and the structure is coarse, which is often coarse overheated structure or coarse hardened structure. Its performance is the worst among welded joints.
What are the factors that affect the performance of welded joints?
A: The influencing factors are: (1) welding material (2) welding method (3) post-weld heat treatment. In addition, the joint form, workpiece thickness, welding ambient temperature and preheating will all affect the cooling rate after welding, thus affecting the microstructure and properties of the joint.
What do 2 1 covered electrode models E4303, E50 15 and covered electrode brands J422 and J507 mean?
Answer: (1) E4303: E means covered electrode coating, 43 means that the tensile strength of deposited metal is 430MPa, 0 means that covered electrode is suitable for metal position welding, 03 means that the welding current type is AC or DC reverse connection, and the coating is titanium.
(2) E50 15 E indicates that the coating covered electrode, 50 indicates that the tensile strength of deposited metal is 500MPa, 1 indicates that the coating covered electrode is suitable for all-position welding, and 15 indicates that the coating covered electrode is DC-reversed low-sodium hydrogen coating.
(3) J422 J indicates that covered electrode is a structural steel covered electrode, 42 indicates that the tensile strength of weld metal is 420MPa, and 2 indicates that covered electrode is a titanium-calcium type coating, which is connected by alternating current and direct current.
(4)J507 J indicates that covered electrode is a structural steel covered electrode, 50 indicates that the tensile strength of weld metal is 500MPa, and 7 indicates that covered electrode is DC reverse connection with low sodium hydroxide coating.
What is the difference between the properties of alkaline covered electrode and acidic covered electrode?
A: Compared with acidic covered electrode, alkaline covered electrode has the following characteristics:
(1) Alkaline covered electrode has good mechanical properties, (2) Alkaline covered electrode has good crack resistance, (3) Alkaline covered electrode has poor process performance, (4) Alkaline covered electrode is sensitive to rust, oil and water, and it is easy to produce pores, and (5) Alkaline covered electrode produces toxic gas and smoke.
23 How to choose structural steel covered electrode? Welding Q235, 20. 45。 16Mn(Q235) What brand of covered electrode should I use?
Answer: (1) The selection method of general structural steel covered electrode is to select covered electrode with the same strength grade according to the principle of tensile strength and "equal strength" of the base material.
(2) Omission
What are the characteristics of automatic submerged arc welding compared with covered electrode arc welding? What is the difference in the scope of application?
Answer: Compared with covered electrode automatic welding, submerged arc welding has the following characteristics:
(1) high productivity and low cost (2) good welding quality and stable quality; (3) Good working conditions, no arc and splash, and light labor intensity; (4) The adaptability of submerged arc automatic welding is poor; (5) The automatic submerged arc welding equipment is complex, which requires a large one-time investment and a large amount of workpiece preparation such as pre-welding test adjustment.
Differences in scope of application:
(1) automatic submerged arc welding is suitable for flat welding and fillet welding of long straight seams and large diameter girth seams of medium and heavy plate structures in mass production.
(2) covered electrode arc welding is suitable for single piece and small batch production. It is used for irregular welds and short welding positions with a thickness of more than 2mm, and for welding where welders can't reach.
What are the characteristics of TIG welding compared with covered electrode arc welding? What is the scope of application?
Answer: Compared with covered electrode arc welding, TIG welding has the following characteristics:
(1) Good mechanical protection effect, pure weld metal, excellent welding quality and beautiful weld formation.
(2) The arc is very stable, especially at low current.
(3) All-position welding is easy to realize automatic control.
(4) Argon gas is more expensive and the cost is higher.
Used for welding oxidizable nonferrous metals and alloy steels, and also suitable for single-sided welding and double-sided forming. In addition, the welding current should not be too large, and only thin plates below 4mm can be welded.
What is solderability? What are the factors that affect solderability?
Answer: The ability of materials to be welded into components according to the specified design requirements and meet the predetermined requirements under limited construction conditions is called weldability. Weldability is influenced by four factors: material, welding method, component type and application requirements.
What are the main problems in welding ordinary low alloy steel and austenitic stainless steel? What are the common welding methods?
A: The main problem in welding ordinary low-alloy steel is cold crack. Covered electrode, arc welding and submerged arc automatic welding are commonly used. In addition, gas shielded welding can also be used. Gas shielded welding can be used for those with lower strength grade.
When the welding material is improperly selected or the welding process is unreasonable, intergranular corrosion and hot cracking will occur, which are two main problems in welding austenitic stainless steel. Covered electrode arc welding and automatic submerged arc welding can also be used for TIG welding.
Castability of alloy: the ability of alloy to easily obtain high-quality castings through liquid forming is called castability.
Cold deformation strengthening: when metal is cold deformed, with the increase of change degree, strength and hardness increase, and plasticity and toughness decrease.
Plastic formability of metal: the difficulty of obtaining qualified parts by plastic forming of metal materials.
Sheet metal stamping: Sheet metal stamping is a processing method to deform or separate the sheet metal by using the die on the press to obtain the blank or parts.
Welding: Welding is a method of joining and processing by heating or pressing (or both). With or without filler, the workpiece is bonded between atoms.
Welding thermal cycle: during the heating and subsequent cooling of welding heat source, the process that the temperature of a certain point on the weld changes with time is called welding thermal cycle.
Brazing: Brazing is a welding method in which a metal material with a melting point lower than that of the base metal is used as the brazing filler metal, the weldment and the brazing filler metal are heated to a temperature higher than that of the transition material and lower than that of the base metal, the base metal is wetted by the liquid brazing filler metal, the joint gap is filled, and the connection is realized by diffusion with the base metal. Weldability of materials: the ability of materials to be welded into components according to the specified design requirements under limited construction conditions and meet the expected use requirements.