A: Rolling angle: the included angle between the radius vectors of the entrance plane and the exit plane. It can be calculated by the thickness of the rolled piece at the entrance and exit and the diameter of the roll.
Rolling force: it is synthesized by all normal stresses and frictional shear stresses acting on the geometric deformation zone, and its direction can be determined by the balance of all forces acting on rolling. In actual production, the vertical component of resultant force is often called "rolling force". Rolling force can be calculated by rolling force formula, and deformation resistance can be multiplied by contact area. Of course, elastic flattening should be considered.
Biting conditions: Unlike almost all other plastic forming methods, the rolling process requires a certain amount of friction, so that the rolled piece will be bitten as soon as it touches the driving roller, and then "dragged" through the geometric deformation zone. When the friction component that plays a drag role in the rolling direction is greater than or equal to the normal component that plays a push-back role, the meshing condition is satisfied.
2. What are the conditions for the completion of scrolling?
Answer: The rolling completion condition is determined according to whether the rolled piece is completely filled with the geometric deformation zone. If it is full, it can be considered that the normal force and friction force act on the center of the geometric deformation zone, and the rolling condition is completed. In other words, this formula satisfies:
3. What are spreading and sliding forward? What are the factors that affect it?
A: 1) Width spread: During rolling, the metal in the deformation zone will not only flow in the longitudinal direction, but also some metal will flow in the width direction, thus changing the width. This phenomenon is called spreading. The influencing factors are:
The increase of height/width ratio increases the width broadening;
The increase of ratio reduction increases the width expansion;
Width broadening increases with the increase of roll diameter d;
The increase of friction coefficient μ increases the spread;
The increased strength reduces the width spread;
The rising temperature θ reduces the width broadening;
Increasing the rolling speed υ will reduce the width broadening.
2) Forward sliding: In the rolling process, the metal flow speed between neutral surface and the outlet of deformation zone will be greater than the speed of the roll, which is called forward sliding. The position of neutral surface affects the forward slip. The closer neutral surface is to the exit, the smaller the forward slip value is.
4. What is the direction and expression of rolling force?
A: The rolling force is obtained by integrating all normal stresses and frictional shear stresses acting on the geometric deformation zone, and its direction can be determined by the balance of all forces acting on the rolling. In actual production, the vertical component of resultant force is often called "rolling force".
Symmetrical rolling, that is, the speed and diameter of two rolls are the same, as shown in the figure.
Force acting on the workpiece during symmetrical rolling
Due to symmetry, the upper and lower rolling forces are mirror images with respect to the rolling line. Force balance is
Vertical direction:
Horizontal direction:
The inclination of rolling force with respect to the vertical line is
If the longitudinal tension is equal, when rolling without longitudinal tension, the rolling force takes the vertical direction or the connecting line parallel to the center of the two rolls.
5. What is the rolling force and give the expression?
A: Rolling force is obtained by the product of rolling torque and roll angular velocity.
Expression:
If the unit of rolling torque is Nm and the unit of rolling speed is min- 1, the formula of rolling power (w) is
6. Is the winding speed the same as the rolling speed? Give the expression of rolling deformation zone speed?
A: The roll speed is different from the rolling speed. The rolling speed is the exit speed of the rolled piece, which is higher than the rolling speed. Only in neutral surface (x=xF) can the speed of the rolled piece be the same as that of the roll (υ=υux).
As shown in figure 1-7. The relative speed of the neutral plane is υ rel =υx-υ UX < 0, and υ rel =υx-υ UX > 0
7. With a roll diameter of 200mm, roll from thick plate h0=2mm to thick plate h 1= 1.4mm, and calculate the rolling angle a0?
Solution: According to △h=2- 1.4=0.6mm, r = 200/2 =100 mm.
Solution, a0=4.4390.
8. What is the relationship between the forward slip value and the position of neutral surface? The forward slip value K 1? is calculated when the roll diameter d=400mm, h0=3mm (thickness before rolling), h 1=2. 1mm (thickness after rolling) and the friction coefficient μ=0.2.
A: The relationship between the forward slip value and the neutral surface position is:
d=400mm，h0=3mm，h 1=2. 1mm，μ=0.2 △h=3-2. 1=0.9mm
Substitute the numerical value into the above formula: a0=3.8440 K 1=5.56.
9. According to the different drawing dies, how many drawing methods are there?
A: There are two kinds, roller drawing and die drawing.
Drawing die means that the workpiece is drawn through an internal drawing die (drawing die, drawing ring and drawing hole), and the drawing die is fixed in the drawing direction. The inside of the mold is called a drawing hole.
Roller pulling is to pull a workpiece through an opening formed by two or more rollers. This can be regarded as a special method, which is of great significance mainly when the die drawing has unique difficulties.
10, give the expression of drawing force? How many parts does the pulling force consist of?
A: tension expression:
Drawing force consists of three parts: ideal quantity part, friction part and shearing part.
1 1, the expression of friction when drawing, and briefly explain the schematic diagram?
Answer: The expression of friction during stretching is
Schematic diagram:
Friction during stretching
The normal force acting on the outer surface AM is (1)
Although the commonly used estimation formula is safe, it is too rough. The outer surface AM is approximated by the geometric shape of the deformation zone as follows
Finally, the following friction forces are obtained by using Formula (2-30) and Formula (2-3 1)
(2)
The friction part in the drawing stress is
(3)
12, what is the drawing angle? Under what conditions does the expression of optimized hole inclination angle hold? What is the tensile angle when the tensile deformation is 0.2 and the friction coefficient is 0.05?
Answer: The drawing angle refers to the opening angle of the drawing hole, which is twice the inclination angle of the drawing hole. The included angle of the extension line of the inner hole surface of the section passing through the axis of the drawing hole. The expression of the optimal drawing hole inclination angle is based on the condition that the drawing force f is much smaller than the force Fmax that damages the drawing part. When the drawing deformation is 0.2 and the friction coefficient is 0.05, the drawing angle is estimated to be 140.
13. Why is the drawing process based on work hardening? Why is drawing generally cold drawing or warm drawing?
A: Drawing is generally cold-drawn or warm-drawn. When metal is deformed at room temperature or low temperature, work hardening is inevitable, so the drawing process is based on work hardening.
14, how to classify extrusion processes? What are forward extrusion and reverse extrusion?
Answer: The extrusion process is divided into direct extrusion and indirect extrusion. Positive extrusion refers to the extrusion method in which the blank moves axially relative to the extrusion cylinder during extrusion; Reverse extrusion means that there is no relative movement between the blank and the extrusion cylinder, and the hollow extrusion rod moves relative to the blank.
15, what is the extrusion ratio? Expression of extrusion ratio and extrusion deformation degree?
Answer: the extrusion ratio is the ratio of the cross-sectional area of the blank to the cross-sectional area of the strip, and the expression is A 0/A 1.
The expression of extrusion deformation degree is
16, what is the extrusion die angle? How is the extrusion dead zone formed? Please explain it with a schematic diagram.
Answer: The extrusion die angle refers to the angle between the boundary surface of the deformation zone/dead zone and the axis of the blank.
Dead zone is formed in the process of extrusion deformation, because the flat die is adopted or the angle of extrusion die is too large, which makes it difficult for the metal with the edge far away from the die hole to enter the die hole, as shown in the dead zone on the right.
17. What are the two parts of extrusion force? And write an expression?
Answer: In the forward extrusion method, the extrusion force can be understood as the sum of the deformation force and friction force between the blank and the extrusion cylinder, and the expression is:
In the reverse extrusion method, there is no friction (no relative motion) between the blank and the extrusion cylinder, and the extrusion force is equal to the deformation force: F=FU.
18, what do you mean by extrusion rod speed and extrusion speed? What are the initial conditions that determine the extrusion speed?
Answer: the extrusion rod speed υ0 refers to the population speed of the blank; The extrusion speed υ 1 refers to the exit speed of the strip.
Deformation zone velocity expression:
Extrusion speed is affected by alloy composition, microstructure characteristics of blank, extrusion mode, complexity of extruded product shape and lubrication conditions.
19. What is the crystal structure of magnesium and what is the main mechanism of plastic deformation? Why is magnesium poor in room temperature plasticity?
Answer: Magnesium has hexagonal crystal structure (HCP);
The main mechanism of plastic deformation is: according to Von Mises yield criterion, generally polycrystalline materials need at least five independent slip systems to start stable plastic deformation.
When deformed at room temperature, only the basal plane of magnesium slips. Only three geometric slip systems and two independent slip systems can be provided, which can not meet the requirements of von Mises criterion. Therefore, magnesium is difficult to deform at room temperature and has poor plasticity.
20. What are the main types of wrought magnesium alloys? What brands of wrought magnesium alloys are commonly used?
A: There are generally two classification criteria for wrought magnesium alloys: the chemical composition of the alloy and whether it can be strengthened by heat treatment. Deformed magnesium alloys can be divided into the following categories according to chemical composition: Mg-Li alloy, Mg-Meng alloy, Mg-Al-Zn-Mn alloy, Mg-Zn-Zr alloy, Mg-RE alloy and Mg-Th alloy.
According to whether it can be strengthened by heat treatment, wrought magnesium alloys can be divided into heat-treated wrought magnesium alloys (such as MB7 and MB 15 alloys) and non-heat-treated wrought magnesium alloys (such as MB 1, MB2, MB3, MB5 and MB8 alloys).
The main domestic wrought magnesium alloy brands are MB 1, MB2, MB3, MB5, MB6, MB7, MB8 and MB 15.
1. What is the effect of alloying elements on the properties of steel?
A: 1. Effect of alloying elements on mechanical properties of steel at room temperature.
2. Effect of alloying elements on high temperature mechanical properties of steel.
3. Effect of alloying elements on mechanical properties of steel at low temperature.
4. Influence of alloying elements on weldability and machinability of steel.
Second, analyze the service conditions and performance requirements of engineering structural steel?
Answer: Engineering structural steel refers to a large class of steel specially used for manufacturing various engineering structural parts, such as bridges, hulls, oil or mine racks, rails, high-pressure vessels, pipelines, building steel structures, etc. It mainly bears various loads and requires high yield strength, good plasticity and toughness to ensure the reliability of engineering structures. Because the working environment is exposed to the atmosphere, the temperature can be as low as -50℃, so it requires low-temperature toughness and atmospheric corrosion resistance. In addition, when engineering structural parts are formed, they often need severe deformation, such as cold bending, stamping and shearing. After forming, it is usually connected by welding, so the steel used for components must also have good weldability and forming manufacturability.
3. Compare the differences between carburized steel and nitriding steel in chemical composition, heat treatment process and properties?
Answer: 1. Carburized steel is an important alloy structural steel. The characteristic of this kind of steel is to obtain specific properties by changing the chemical composition of the steel surface to meet the requirements of use. The matrix carbon content (mass fraction) of carburized steel is generally in the range of 0. 15% ~ 0.25%. The carbon content (mass fraction) of individual steel grades is also 0.25% ~ 0.35%, but the carbon content of important parts and parts with large impact load is generally below 0.20%. For general parts, the carbon content of carburized layer is generally 0.8% ~ 1. 1%.
2. Heat treatment of carburized steel: after carburizing, quenching and low-temperature tempering must be carried out to achieve the expected purpose. Common heat treatment methods of carburized steel are as follows:
Figure 3- 16 Common Heat Treatment Process Curve of Carburized Steel
(1) direct quenching. Generally, the parts are precooled from carburizing temperature to about 840℃, then quenched with oil (or water), and then tempered at low temperature to eliminate quenching stress and reduce brittleness.
(2) primary quenching. For parts with high requirements or basically fine-grained steel, one-time quenching after carburizing, low-temperature tempering, air cooling or slow cooling after carburizing, reheating quenching and finally low-temperature tempering can be adopted.
(3) secondary quenching. For parts with high performance requirements, secondary quenching should be adopted after carburizing, and the heating temperature of primary quenching should be above Ac3 (850℃ ~ 900%) in the center, so as to refine the structure of the center, improve the performance of the center and eliminate the network carbide on the surface.
3. Performance requirements of carburized steel: carburized steel requires that the core material has necessary strength and appropriate toughness, and at the same time, it should also have sufficiently high surface residual compressive stress. Not only a hard and wear-resistant surface layer is required to avoid scrapping due to premature wear or contact fatigue damage, but also the surface of the core material is required to have high strength and good toughness, which is the most important technical measure to meet this requirement.
Nitriding steel, also known as nitriding, is one of the important processes in chemical heat treatment. The steel grade suitable for nitriding process is called nitriding steel, also known as nitriding steel. Nitriding is a chemical heat treatment in which active nitrogen atoms penetrate into the steel surface to form a nitrogen-rich hardened layer. Compared with carburizing, after nitriding, higher surface hardness (950 HV ~ 1200 HV, equivalent to 65 ~ 72 rC) and wear resistance can be obtained, which can be maintained at 560℃ ~ 600℃, so the thermal stability of nitriding steel is high.
The chemical composition of nitriding steel is characterized by adding some alloying elements on the basis of medium carbon steel to improve its nitriding performance and other mechanical properties.
Considering the technological performance of nitriding, it is required to obtain a nitrided layer with high hardness, satisfactory brittleness and sufficient depth in the shortest possible time. Nitride-forming elements form ultramicroscopic nitride particles in the instrument phase, which only strengthens this phase. The finer the nitride formed, the greater the effect of dispersion strengthening. These elements include aluminum, chromium, vanadium, II, molybdenum, manganese and tungsten. The steel used for nitriding treatment is generally medium carbon structural steel. Nitriding treatment has the best effect when steel contains alloying elements such as aluminum, chromium and molybdenum.
Because the nitrided steel has the above characteristics in performance, and the processing temperature is low (500℃ ~ 560℃), the cooling after processing is slow and the deformation is small. After nitriding, there is no need for mechanical processing, just fine grinding and polishing, so nitriding treatment has been widely used.
Parts that work under alternating load and require both high fatigue strength and wear resistance, as well as parts that require accurate size, small heat treatment deformation, heat resistance, corrosion resistance and wear resistance, all need nitriding treatment. The main disadvantages of nitriding are long time consuming, high cost, thin and brittle nitriding layer and no impact resistance.
4. What is red hardness? How to measure? Why is it an important performance index of high speed steel?
A: (1) Red hardness refers to the ability of a material to maintain its hardness after a certain period of time at a certain temperature, such as high speed steel.
(2) How to measure?
After normal tempering, the hardness of high speed steel is 63 ~ 63~66HRC, and the microstructure is tempered martensite+carbide+a little retained austenite.
(3) Why is it an important performance index of high speed steel?
There are a large number of coarse carbides with uneven distribution in the as-cast structure of high-speed steel, which has great influence on the quality and service life of high-speed steel tools. When quenching and heating, the coarse carbide dissolves less, which reduces the alloying degree in austenite, reduces the hardness, tempering resistance and wear resistance of the tool after heat treatment, and the bending strength and deflection of high-speed steel also decrease with the increase of carbide unevenness. Therefore, the uniform distribution of carbides is one of the main technical quality indexes for evaluating high-speed steel.
5. What are the ways to improve the corrosion resistance of metals (alloys) by alloying?
Answer: Solving the corrosion problem in engineering can improve the corrosion resistance of stainless steel itself, reduce the corrosion of environmental media and improve defects. In terms of improving the corrosion resistance of stainless steel itself, there are the following methods:
(1) Improve the electrode potential of the stainless steel matrix or form a stable passivation zone to reduce the electromotive force of the primary battery.
(2) Steel has a single-phase structure, and the number of microbatteries is reduced.
(3) forming a stable surface protective film on the steel surface, such as adding silicon, aluminum and chromium. It can form a dense protective film in many corrosion and oxidation occasions and improve the corrosion resistance of steel.
(4) Adopt mechanical protection measures and surface coating, such as electrochemical coating, bluing and painting.
6. Using dislocation theory to analyze the three stages of creep?
A: Under the condition of high-temperature creep, due to thermal activation, it is possible for dislocation climb on the slip surface to form a small-angle subgrain boundary (that is, multilateralization in the high-temperature recovery stage), which leads to the softening of metal materials and the continuation of slip. Under the condition of high temperature creep, due to the decrease of grain boundary strength, its deformation is large, sometimes even accounting for half of the total creep deformation, which is one of the characteristics of creep deformation. In the first stage of creep, deformation occurs in the form of intragranular slip and grain boundary slip. At the initial stage of dislocation movement, there are fewer obstacles and the crawling speed is faster. Subsequently, dislocations are gradually blocked, dislocation density increases, and lattice distortion increases, leading to deformation strengthening. At high temperature, although dislocations can be recovered and softened by climbing to form subgrain, the driving force of dislocation climb comes from the reduction of lattice distortion energy. At the initial stage of creep, the recovery softening process is not obvious because of the small lattice distortion energy. In the second stage of creep, intragranular deformation is replaced by dislocation slip and climbing, and grain boundary deformation is alternated by slip and migration. Intragranular slip and grain boundary slip strengthen the metal, but dislocation climb and grain boundary migration soften the metal. Due to the alternating action of strengthening and softening, the creep speed remains unchanged when equilibrium is reached. Creep develops to the third stage, and the creep speed is accelerated due to the rapid expansion of cracks. Creep fracture occurs when the crack reaches the critical size.