01

Weldability and test evaluation

1. Welding: By heating or pressurizing, with or without filling materials, the process of combining two objects between atoms to form an inseparable whole.

2. Weldability: refers to the ability of homogeneous materials or heterogeneous materials to weld a complete joint under manufacturing process conditions and meet the requirements of intended use.

3. The four major factors affecting weldability are: material, design, process and service environment.

4. The principles of evaluating weldability mainly include: (1) Evaluating the tendency of welding joints to produce process defects to provide a basis for developing reasonable welding processes; ② Assess whether the welded joint can meet the requirements of structural performance; The design of a new welding test method conforms to the following principles: comparability, pertinence, reproducibility and economy.

5. Carbon equivalent: The content of alloying elements in steel is converted and superimposed according to the equivalent of several carbon contents, which is used as a parameter index for roughly evaluating the cold crack tendency of steel.

6. Inclined Y-groove butt crack test: The purpose is mainly used to identify the first layer of low-alloy high-strength steel weld and HAZ cold crack tendency, and can also be used to draft welding process. 1) Specimen preparation, welded steel plate thickness δ=9-38mm. The bevel of the butt joint is processed by mechanical method, and both ends of the test plate are welded with constrained welds within the range of 60mm. Take care to prevent Angle distortion and underpenetration. Ensure that there is a 2mm gap in the weld of the middle specimen to be welded. 2) Test conditions: the electrode selected for the test weld is matched with the base material, the electrode used should be strictly dried, the diameter of the electrode is 4mm, the welding current is (170±10) A, the welding voltage is (24±2) V, and the welding speed is (150±10) mm/min. The test weld can be welded at a variety of different temperatures, and the test weld is only welded without filling the groove. After static welding and natural cooling for 24h, the samples were intercepted and cracks were detected. 3) Detection and calculation of crack strip rate. Use the naked eye or a hand-held 5-10 magnifying glass to detect cracks on the surface and section of the weld and heat affected zone. It is generally believed that when the surface crack rate of the "small iron lapping" test of low alloy steel is less than 20%, there is generally no crack.

7. Pin test: Objective, mainly to assess the hydrogen induced delayed crack tendency of steel, additional equipment, can also determine the reheat crack sensitivity and laminar sensitivity. 1) The specimen is prepared, the welded steel is processed or the cylindrical latch test rod is sampled along the rolling direction and the position of the latch in the thickness direction is indicated. There is a ring or spiral notch near the upper end of the test rod. Insert the latch test rod into the corresponding hole in the bottom plate so that the notched end is flush with the surface of the bottom plate. For the pin test rod with a ring notch, the distance A between the notch and the end face should make the weld penetration depth tangent or intersect with the truncated plane of the notch root, but the portion of the circumference of the notch root that is melted shall not exceed 20%. For low alloy steels, the a value is 2mm when the welding heat input is E=15KJ/cm. 2) During the test process, according to the selected welding method and strictly controlled process parameters, a layer of surfacing weld pass is fused on the bottom plate. The center line of the weld pass passes through the center of the sample, and the penetration depth should be so that the notch tip is located in the coarse crystal zone of the heat affected zone. The length of the weld pass L is about 100-150mm. When welding, the cooling time value of 800-500℃ t8/5 value should be measured, and when welding is not preheated, it should be loaded when cooling to 100-150℃ after welding; When preheating before welding, it should be loaded at 50-70℃ higher than the preheating temperature. The load should be applied within 1min and before cooling to 100℃ or 50-70℃ above the preheating temperature. If there is post-heat, it should be loaded before post-heat. When the test rod is loaded, the latch may break during the duration of the load. Note the load time.

02

Weldability of alloy structural steel

1. High-strength steel: Strength steel with yield strength σs≥295MPa can be called high-strength steel. '

2 Mn solid solution strengthening effect is very significant, ωMn≤1.7%, can improve toughness, reduce the brittle transition temperature, Si will reduce plasticity, toughness, Ni both solid solution strengthening and at the same time improve toughness and greatly reduce the brittle transition temperature of the element, often used in low temperature steel.

3. Hot rolled steel (normalizing steel) : low-alloy high-strength steel with yield strength of 295-490MPa, which is generally supplied in hot rolled or normalizing state.

4. Design principle of high-strength steel welded joint: high-strength steel with its strength as the basis for selection, so the principle of welded joint is: the strength of the welded joint is equal to the strength of the base material (principle of equal strength), analysis: ① the strength of the welded joint is greater than the strength of the base material, the plastic toughness is reduced, ② is equal to when the life is quite ③ less than when the joint strength is insufficient.

5. Weldability of hot rolled and normalizing steel: hot rolled steel contains a small amount of alloying elements in general, cold crack tendency is not large, normalizing steel due to more alloying elements, hardening tendency has increased, with the increase of normalizing steel carbon equivalent and plate thickness, hardening and cold crack tendency increases. Influencing factors: (1) carbon equivalent (2) Hardening tendency: the hardening tendency of hot-rolled steel and the hardening tendency of normalizing steel (3) The maximum hardness of the heat-affected zone, the maximum hardness of the heat-affected zone is a simple method to evaluate the hardening tendency and cold crack susceptibility of steel.

6.SR crack (stress relief crack, reheat crack) : Welded structures such as Mo normalizing steel thick wall pressure vessels may have another form of crack during post-welding stress relief heat treatment or post-welding reheating at high temperature.

7. Toughness is a property that characterizes the metal's difficulty in generating and expanding brittle cracks.

8, low alloy steel selection of welding materials must consider two aspects of the problem: ① can not have cracks and other welding defects ② can meet the performance requirements. The welding material of hot-rolled steel and normalizing steel is generally selected according to its strength level, and its selection points are as follows: (1) Select the corresponding level of welding material that matches the mechanical properties of the base metal; (2) consider the influence of fusion ratio and cooling rate; (3) consider the influence of post-welding heat treatment on the mechanical properties of the weld.

9. Determine the principle of tempering temperature after welding: ① do not exceed the original tempering temperature of the base material so as not to affect the performance of the base material itself ② For tempered materials, to avoid the temperature range of tempering brittleness.

10. Tempered steel: quenching + tempering (high temperature).

11. High strength steel welding using "low strength matching" can improve the crack resistance of the welding area.

12. Two basic problems should be paid attention to when welding low-carbon tempered steel: (1) The cooling rate of martensite transformation is not too fast, so that martensite has self-tempering effect to prevent the generation of cold cracks; (2) The cooling rate between 800 ° C and 500 ° C is greater than the critical speed of producing brittle mixed tissue. The problems to be solved in the welding of low-carbon tempered steel are: ① to prevent cracks; ② to improve the toughness of weld metal and heat affected zone while ensuring that high strength requirements are met.

13. For low alloy steel with low carbon content, increasing the cooling speed to form low carbon martensite is beneficial to ensure toughness.

14. The addition of alloy elements of medium carbon tempered steel mainly plays a role in ensuring hardenability and improving tempering resistance, and the true strength performance mainly depends on the carbon content. Main features: high specific strength and high hardness.

15. There are three ways to improve the thermal strength of pearlite heat-resistant steel: ① matrix solid solution strengthening, adding alloying elements to strengthen the ferritic matrix, commonly used Cr,Mo,W,Nb elements can significantly improve the thermal strength ② second phase precipitation strengthening: in the heat-resistant steel with ferritic matrix, the strengthening phase is mainly alloy carbide ③ grain boundary strengthening: The addition of trace elements can adsorb the grain boundary, delay the diffusion of alloying elements along the grain boundary, and strengthen the grain boundary.

16. The main problems in the welding of pearlite heat-resistant steel are cold cracking, hardening and softening of the heat-affected zone, and stress cracking in post-welding heat treatment or long-term use at high temperatures.

17. The temperature range from -10 to -196 ° C is called "low temperature", and below -196 ° C is called "ultra-low temperature".

03

Stainless steel welding

1. Stainless steel: Stainless steel is a general term for alloy steel with high chemical stability that is resistant to air, water, acid, alkali, salt and its solution and other corrosive media.

2. The main corrosion forms of stainless steel are uniform corrosion, point corrosion, gap corrosion and stress corrosion. Uniform corrosion refers to the phenomenon that the metal surface in contact with the corrosive medium all produces corrosion; Point corrosion refers to the local corrosion that occurs when most of the surface of the metal material is not corroded or corroded slightly. Gap corrosion: In the electrolyte, such as in the oxygen ion environment, when there is a gap between the stainless steel or the surface in contact with the foreign body, the flow of the solution in the gap will be hysteresis phenomenon, so that the local Cl- of the solution will form a concentrated battery, resulting in the adsorption of Cl- by the stainless steel passivation film in the gap and the phenomenon of local destruction; Intergranular corrosion, selective corrosion near grain boundaries; Stress corrosion refers to the phenomenon of brittle cracking of stainless steel below the extremely strong strength under the action of specific corrosive media and tensile stress.

3. Measures to prevent point corrosion: 1) reduce the chloride ion content and oxygen ion content 2) add chromium, nickel, molybdenum, silicon, copper and other alloying elements in stainless steel 3) try not to carry out cold processing to reduce the possibility of point corrosion at the dislocation outcrop 4) reduce the carbon content in steel.

4. The high temperature performance of stainless steel and heat-resistant steel: 475℃ brittleness, mainly in Cr > 13% ferrite, long-term heating and slow cooling between 430-480℃, resulting in increased strength and decreased toughness at room temperature or negative temperature; σ-phase embrittlement is typical of 45% of the mass fraction of Cr, FeCr intermetallic compound, non-magnetic, hard and brittle.

5. Corrosion resistance of austenitic stainless steel welded joints: 1) intergranular corrosion, 2) intergranular corrosion in heat-affected zone sensitized zone, 3) knife corrosion.

6. Measures to prevent intergranular corrosion of welds: 1) By welding materials, make the weld metal either ultra-low carbon condition, or contain enough stabilizing element Nb. 2) Adjust the weld composition to obtain a certain δ phase. The theory of intergranular corrosion is essentially the theory of chromium deficiency.

7. Intergranular corrosion in the sensitized zone of the heat affected zone: refers to the intergranular corrosion in the welding heat affected zone where the peak heating temperature is in the sensitized heating interval.

8. Knife-like corrosion: The intergranular corrosion produced in the fusion zone is like the form of knife cutting, so it is called "knife-like corrosion".

9. To prevent knife corrosion measures: ① the selection of low carbon base material and welding materials ② the use of phase structure of stainless steel ③ the use of small current welding, reduce the degree of overheating and width of the welding coarse crystal area ④ and the corrosion medium contact weld final welding ⑤ cross welding ⑥ increase the Ti,Tb content in the steel, so that the grain boundary of the welding coarse crystal area has enough Ti,Tb and carbonization.

10. Why use small current welding of stainless steel? In order to reduce the temperature of the welding heat affected zone, prevent the occurrence of intercrystalline corrosion of the weld, prevent the welding rod, welding wire overheating, welding deformation, welding stress, can reduce the heat input.

11. Three conditions that cause stress corrosion cracking: environment, selective corrosive media, tensile stress.

12. Measures to prevent stress corrosion cracking: 1) adjust the chemical composition, ultra-low carbon is conducive to improving the ability to resist stress corrosion, the matching problem between the composition and the medium, 2) remove the welding residual stress, 3) electrochemical corrosion, regular inspection and timely repair.

13. In order to improve pitting resistance: 1) On the one hand, Cr and Mo segregation must be reduced; 2) On the other hand, so-called "superalloying" welding materials with higher Cr and Mo content than the base material are used.

14. Austenitic stainless steel welding will produce hot cracks, stress corrosion cracks, welding deformation, intergranular corrosion.

15. The reasons for welding hot cracks of austenitic steel are as follows: 1) Austenitic steel has small thermal conductivity, large linear expansion coefficient, and large tensile stress; 2) Austenitic steel is easy to combine crystallization to form a strong directional column crystal weld structure, which is conducive to the segregation of harmful impurities; 3) austenitic steel alloy composition is more complex and easy to dissolve eutectic.

16. Measures to prevent hot cracking: ① strictly limit the P and S content in the base metal and welding materials ② try to make the weld form a two-phase structure ③ control the chemical composition of the weld ④ small current welding.

17.What is the difference in weld structure between type 18-8 and type 25-20 when preventing thermal cracking? 18-8 steel weld formed A+δ structure, δ phase can dissolve A large number of P,S, δ phase is generally 3%-7%, 25-20 steel weld formed A+ primary carbide structure.

18. Austenitic stainless steel selection should pay attention to: ① Adhere to the "applicability principle" ② Determine whether it is suitable according to the specific composition of the selected welding materials ③ consider the welding method and process parameters of the specific application may cause the size of the fusion ratio ④ determine the degree of alloying according to the comprehensive weldability requirements specified in the technical conditions ⑤ pay attention to the weld metal alloy system, the role of the specific alloy composition in the alloy system. Consider service performance requirements and process weldability requirements.

19. Weldability analysis of ferritic stainless steel: 1) intergranular corrosion of welded joints 2) embrittlement of welded joints, high temperature embrittlement, σ embrittlement, 475℃ embrittlement.

04

Cast iron welding

1. Three characteristics of cast iron: vibration reduction, oil absorption, wear resistance.

2. The performance of cast iron mainly depends on the shape, size, quantity and distribution of graphite, and the matrix structure also has a certain impact.

3. Ductile iron: F matrix + spherical graphite; Gray cast iron: F matrix + flake graphite; Vermicular graphite cast iron: matrix + vermicular graphite; Malleable cast iron: F matrix + flocculent graphite.

4. Can low carbon steel welding rod weld cast iron? No, in welding, even if the current is small, the base material in the first weld accounted for 25%-30%, if calculated according to C=3% in cast iron, the carbon content in the first weld is 0.75%-0.9%, belonging to high carbon steel, high carbon martensite immediately after welding cooling, and welding HAZ will appear white tissue, mechanical processing difficulties.

5. Arc welding: the molten cast is preheated to 600-700 ° C, and then welded in the plastic state, the welding temperature is not less than 400 ° C, in order to prevent cracking during the welding process, immediately after welding stress relief treatment and slow cooling, this cast iron welding repair process is called arc welding.

6. Semi-thermal welding: When the preheating temperature is 300-400℃, it is called semi-thermal welding.以上翻译结果来自有道神经网络翻译（YNMT）· 通用场景