Weldability of metallic materials
- Weldability of metals:
Metal weldability refers to the ability of homogeneous or heterogeneous materials to form a solid joint and meet desired performance requirements during the manufacturing process. There are two types of weldability: process weldability and service weldability.
- Process weldability:
Process weldability is the ability of a metal or material to produce high-quality, dense, defect-free welded joints that meet performance requirements under specific welding process conditions.
- Weldability:
Weldability refers to the extent to which the welded joint and the overall welded structure meet various properties, including conventional mechanical properties.
- Factors influencing metal weldability:
There are four factors that can affect the weldability of metal: material factor, design factor, process factor and service environment.
- Weldability assessment principles:
To evaluate weldability, the following principles should be considered: (1) Evaluate the probability of process defects in welded joints to provide a basis for designing a suitable welding process. (2) Evaluate whether the welded joint meets structural performance requirements.
- Principles for experimental methods:
Experimental methods must meet the following principles: comparability, relevance, reproducibility and economy.
- Common weldability test methods:
A. Oblique V-groove welding crack test method: This method is mainly used to evaluate the sensitivity of the heat-affected zone of welding carbon steel and low-alloy steel and high resistance to cold cracking.
B. Pin Test
C. Butt welding crack test method for pressing plate
D. Adjustable restraint crack test method
I. Questions and answers:
1. What is the purpose of the experiment and to what occasion does it apply?
Understand the main stages of the experiment and analyze the factors that impact the stability of the results.
To respond:
The objective is to evaluate the vulnerability of the thermally affected zone when welding carbon steels and low-alloy steels with high resistance to cold cracking.
When determining the sensitivity of the heat-affected zone when welding carbon steel and low-alloy steel and high resistance to cold cracking, the factors that influence the stability of the results are the restraint of the welded joint, the preheat temperature, deformation Angular and incomplete penetration.
It is commonly accepted that if the surface cracking rate in low alloy steel is less than 20%, it is considered safe for general welding structures.
2. What are the main factors that affect process weldability?
Answer: influencing factors:
(1) Material Factors: Covers the base metal and welding materials used, including welding rods for electrode arc welding, welding wires and fluxes for submerged arc welding, welding wires and shielding gases for shielded welding. gas, among others.
(2) Design Factors: The design of welded joint structures will impact the state of stress, thereby affecting weldability.
(3) Process Factors: Even for the same base metal, different welding methods and process parameters can have a significant impact on weldability.
(4) Service environment: The service environment for a welded structure may vary, such as working temperature, type of working medium and load properties, among others.
3. Sometimes metal materials with good process weldability may not have good use weldability.
To respond:
The use and welding properties of metallic materials refer to the various properties specified by the technical requirements of the welded joint or the overall welded structure, including conventional mechanical properties or properties under specific working conditions, such as low temperature toughness, fracture toughness , high-temperature creep resistance, long-term resistance, fatigue performance, corrosion resistance and wear resistance.
The weldability of a process refers to the ability of a metal or material to produce high-quality, dense, defect-free, and functional welded joints under specific welding process conditions.
For example, low carbon steel has good weldability, but its strength and hardness are not as high as high carbon steel.
4. Why can the higher hardness of the heat-affected zone be used to evaluate the cold cracking sensitivity of welding iron and steel materials? What is the effect of welding process conditions on the maximum hardness of the heat affected zone?
To respond:
(1) Cold cracks normally occur in the heat-affected zone;
(2) Assessment of joint hardness is the most crucial factor in determining the probability of cold cracking, making it a useful indicator.
Typically, the welded joint includes the heat-affected zone.
The greater the difference between the hardness value of the welded joint and the base metal, the lower the joint's toughness and overall mechanical properties, making it more susceptible to brittle fracture and other hazards.
To minimize this difference and ensure the reliability of the welded joint, the welding process conditions must be carefully controlled.
Although an increase in carbon equivalent generally leads to an increase in heat-affected zone hardening, this relationship is not always linear.
2. Welding of structural steel alloy
1. Weldability analysis of low carbon quenched and tempered steels
Low carbon quenched and tempered steel is mainly used as high-strength welded structural steel with low carbon content limit. The alloy composition is designed keeping weldability requirements in mind. The carbon content in low carbon quenched and tempered steel is less than 0.18%, resulting in better welding performance compared to medium carbon quenched and tempered steel.
The low carbon martensite in the welding heat affected zone of this steel results in a high martensite transformation temperature (MS) and self-tempering martensite, leading to a lower tendency for cold crack welding compared to quenched and tempered steel medium carbon. Good toughness can be achieved when fine structures of low-carbon martensite (ML) or lower bainite (B) are obtained in the heat-affected zone.
The mixed structure of ML and low-temperature transformed bainite (B) provides the best toughness, with distinct crystalline positions between the bainite laths. The effective grain diameter is fine and has good toughness and depends on the strip width. The mixture of ML and BL effectively splits the original austenite grains, promoting more nucleation positions for ML and limiting its growth. The effective grains in the ML + B mixed structure are the smallest.
Ni is an important element in the development of low-temperature steel and its addition can improve the low-temperature properties of steel. For example, 1.5Ni steel must have a reduced carbon content and strict limits on S, P, N, H and O content to avoid aging brittleness and tempering brittleness while increasing Ni. Heat treatment conditions for this type of steel include normalizing, normalizing + tempering, and quenching + tempering.
In low-temperature steel, strict control of carbon content and impurities such as S and P reduces the likelihood of liquefaction cracks. However, temper brittleness can still be a concern, and it is important to control the tempering temperature and cooling rate after welding.
Features of low temperature steel welding process:
The main objective in welding low-temperature steel is to maintain the low-temperature toughness of both the weld and the heat-affected zone in order to prevent cracking.
9Ni steel has strong toughness at low temperatures, but when welding with ferritic materials similar to 9Ni, the toughness of the weld is greatly reduced.
This can be attributed to the microstructure of the molten solder and the oxygen content in the solder.
However, 11Ni ferritic welding materials, which are similar to 9Ni steel, can achieve good toughness at low temperatures through TIG welding. This is because TIG welding reduces the oxygen content in the weld metal to less than 0.05% of the base metal.
2. Weldability analysis of quenched and tempered medium carbon steels
Hot cracks in carbon quenched and tempered steel welds are often caused by high carbon and alloy content, which results in a large liquid-solid gap and severe segregation. These factors increase the likelihood of hot cracking.
Cold cracks in quenched and tempered medium carbon steels are caused by the high carbon content and abundance of alloying elements, which result in a hardening tendency. Furthermore, the low melting point of steel results in the formation of martensite at low temperatures, which does not have the ability to self-temper and increases the likelihood of cold cracking.
Reheating of cracks in the heat-affected zone may result in changes in performance.
Embrittlement in the superheated zone
(1) Medium carbon quenched and tempered steel has high carbon content, various alloying elements and strong hardenability, making it susceptible to the production of hard and brittle martensite with high carbon content in the superheated welding zone. The faster the cooling rate, the greater the formation of high-carbon martensite and the more pronounced the tendency to embrittlement.
(2) Despite the high linear energy, it can be challenging to avoid the formation of high-carbon martensite, which results in coarser and more brittle material.
(3) To improve the performance of the superheated zone, measures such as linear low power, preheating, slow cooling and postheating are typically employed.
Softening of the heat-affected area
When a quenching and tempering treatment is not possible after welding, it is necessary to take into account the softening of the heat-affected zone. The stronger the type of quenched and tempered steel, the more serious the softening problem becomes. The length and width of the softening zone are closely linked to the linear energy and the method used in welding.
3. Characteristics of the welding process of quenched and tempered medium carbon steel
(1) In weld hot cracking, the carbon and alloy element content of carbon quenched and tempered steel is high, leading to a large liquid-solid gap, severe segregation and a high tendency to hot cracking.
(2) Cold cracking in quenched and tempered medium carbon steels is caused by their high carbon content and greater presence of alloying elements, resulting in an evident tendency to harden.
(3) The low melting point results in the formation of martensite at low temperatures that generally does not have the ability to self-temper, leading to a high tendency to cold cracking.
(4) Performance changes in the heat-affected zone.
Embrittlement in the superheated zone
(1) Medium carbon quenched and tempered steel is prone to producing hard and brittle martensite with high carbon content in the superheated welding zone due to its high carbon content, numerous alloying elements and significant hardenability. The faster the cooling rate, the more high-carbon martensite will be formed and the more severe the embrittlement tendency will become.
(2) Despite having high linear energy, it is a challenge to avoid the formation of martensite with high carbon content, which will make the material coarser and brittle.
(3) To improve the performance of the superheated zone, measures such as low linear power, preheating, slow cooling and postheating are generally employed.
Softening of the heat-affected area
When welding is completed and quenching and tempering treatment cannot be carried out, it is necessary to take into account the softening of the heat affected zone (HAZ).
The more the degree of strength of quenched and tempered steel increases, the more pronounced the softening problem becomes.
The extent and width of softening are closely linked to the welding line energy and the welding method used.
The welding method that uses a more concentrated heat source is more advantageous in reducing softening.
4. Characteristics of the welding process of quenched and tempered medium carbon steel
(1) Medium carbon quenched and tempered steel is normally welded in its annealed state. After completion of the welding process, uniform welded joints with desirable properties can be achieved through a general quenching and tempering treatment.
(2) When welding is performed after quenching and tempering, it is often challenging to address the performance degradation of the heat-affected zone.
(3) The pre-welding state determines the nature of the problems and the necessary steps to be taken in the process.
The weldability characteristics of Q345 steel are analyzed, and the corresponding welding materials and welding process requirements are provided.
Answer: Q345 steel is a type of hot-rolled steel with a carbon content of less than 0.4% and excellent weldability.
Generally, preheating and precise control of the welding heat input are not necessary. However, it is important to consider the potential effects on the material.
Regarding brittle and hard properties, when Q345 steel is continuously cooled, the pearlite transformation shifts to the right, resulting in ferrite precipitation under rapid cooling, leaving the carbon-rich austenite to transform into pearlite too late. This transformation into high-carbon bainite and martensite leads to a hardening effect. However, due to its low carbon content and high manganese content, Q345 steel has good resistance to hot cracking.
By adding V and Nb to Q345 steel, stress cracking in the welded joint can be eliminated through precipitation strengthening.
It is important to note that embrittlement of coarse grains can occur in the superheated zone of the heat-affected zone when heated above 1200℃, resulting in a significant reduction in toughness. However, annealing Q345 steel at 600℃ for 1 hour greatly improves its toughness and reduces the tendency to thermal deformation embrittlement.
For the selection of welding material, the following options are recommended:
- Butt welding electrode: E5 series
- Arc welding electrode: E5 series
- Submerged arc welding: SJ501 flux, H08A/H08MnA welding wire
- Electroslag welding: HJ431, HJ360 flux, H08MnMoA welding wire
- CO2 gas shielded welding: H08 series and YJ5 series
It is recommended to preheat the material to a temperature of 100 to 150 ℃. For post-welding heat treatment, arc welding typically does not require it, or can be tempered to 600 to 650 ℃. Electroslag welding, on the other hand, requires normalizing at 900 to 930℃ and tempering at 600 to 650℃.
What is the difference in weldability between Q345 and Q390? Is the Q345 welding process applicable to Q390 welding and why?
Answer: Q345 and Q390 are hot-rolled steels with similar chemical composition.
The only difference between Q345 and Q390 is in the Mn content, with Q390 having a higher concentration. As a result, the Q390 has a higher carbon equivalent compared to the Q345.
This results in greater hardenability and greater probability of cold cracking in Q390 when compared to Q345. However, their weldability remains similar.
It should be noted that the welding process used for Q345 may not be suitable for Q390 due to its higher carbon equivalent and higher heat input, which could result in overheating and severe embrittlement in the joint area if the heat input is too loud, or cold. cracking and brittle behavior if heat input is too low.
What is the selection principle of welding materials when welding low-alloy and high-strength steel? What is the effect of post-weld heat treatment on welding materials?
Answer: The selection principle should take into account the impact of the weld microstructure and the heat-affected zone on the strength and toughness of the welded joint.
Since post-weld heat treatment is generally not performed, it is crucial that the weld metal has mechanical properties similar to those of the base metal in its welded state.
For medium-carbon quenched and tempered steel, the choice of welding materials should be based on the weld stress conditions, its performance requirements, and any planned post-weld heat treatment.
For components that will undergo treatment after welding, the chemical composition of the weld metal must be comparable to that of the base metal.
Analyze possible problems when welding quenched and tempered steels with low carbon content.
This post provides a brief overview of the key aspects of welding low carbon quenched and tempered steel.
What is the recommended range for controlling the welding heat input of typical low carbon quenched and tempered steels such as 14MnMoNiB, HQ70 and HQ80?
When preheating is necessary, why are there minimum temperature requirements and how can the maximum preheating temperature be determined?
Answer: Embrittlement can easily occur during the welding process. Thermal cycling during welding can reduce the strength and toughness of the heat affected zone.
Welding process features: Normally, post-welding heat treatment is not required. A multilayer process is used and a narrow weld bead is employed instead of the oscillating strip transverse conveying technique.
The welding heat input for typical low-carbon quenched and tempered steel should be controlled to be less than 0.18% WC, and the cooling rate should not be accelerated. When the WC is greater than 0.18%, the cooling rate can be increased to reduce heat input.
The welding heat input must be kept below 481 kJ/cm. If the maximum allowable welding heat input is reached and cracking cannot be avoided, preheating measures must be taken.
If the preheating temperature is too high, it will not prevent cold cracks from occurring. On the other hand, if the cooling rate between 800 and 500°C is slower than the critical cooling rate of brittle composite structures, the toughness of the heat-affected zone will decrease.
Therefore, it is important to avoid unnecessary increases in preheating temperature, even at room temperature. As a result, there is a minimum preheating temperature.
The maximum allowable heat input for welding steel should be determined through experiments and then, based on the tendency of cold cracking at maximum heat input, it should be decided whether to preheat and preheat temperature, including the maximum preheat temperature are required.
What is the difference in the welding process between quenched and tempered and annealed quenched and tempered medium carbon steels of the same brand? Why are medium carbon quenched and tempered steels generally not welded in the annealed state?
When welding in the quenched and tempered state, it is crucial to follow proper procedures to avoid delayed cracking and eliminate the hardened structure in the heat-affected zone. This includes preheating, maintaining control of temperatures between passes, conducting intermediate heat treatment, and timely tempering after welding.
To minimize the softening of the thermal effect, it is recommended to adopt a method with high energy density and heat concentration, and use the lowest possible welding heat input.
For welding in the annealed state, common welding methods can be employed.
When selecting materials, it is important to ensure consistency in quenching and tempering treatment specifications of the weld metal and base metal, as well as consistency in its parent alloy.
In the case of quenching and tempering, a high preheat temperature and interlayer temperature can help prevent cracking before treatment.
Due to the high hardenability and hardenability of medium carbon quenched and tempered steel, improper welding in the annealed state may result in delayed cracking.
A complex welding process is typically required, and auxiliary processes such as preheating, postheating, tempering, and postweld heat treatment can help ensure joint performance and longevity.
Is there any difference in the welding process and material selection when low temperature steel is used at –40℃ and at normal temperature? Why?
Answer: To avoid low-temperature embrittlement and thermal cracking in low-temperature steel welded joints, it is important to minimize the presence of impurity elements in the materials.
To control the composition and structure of the weld, it is important to select appropriate welding materials that will form fine acicular ferrite and a small amount of carbide alloy, thus ensuring certain AK requirements at low temperatures.
When using SMAW (Shielded Metal Arc Welding) in low temperature welding, the use of small linear energy welding can avoid overheating of the heat affected zone and reduce the formation of coarse M and WF (Weld Fracture). To further reduce weld bead overheating, multi-pass rapid welding can be applied.
For the SAW (Submerged Arc Welding) process, using the vibrating arc welding method can prevent the formation of columnar crystals.
What are the differences in reinforcement methods and main reinforcement elements between hot-rolled steel and standard steel, and what are the differences in weldability between them? What problems should we pay attention to in the formulation of the welding process?
Answer: Hot rolled steel reinforcement methods are:
(1) Solid Solution Strengthening: The main strengthening elements in this process are Mn and Si.
(2) Fine-grain strengthening: The main strengthening elements in this process are Nb and V.
(3) Precipitation Strengthening: The main strengthening elements in this process are Nb and V.
Standardized steel reinforcement mode:
Weldability: Hot-rolled steel contains a limited number of alloying elements and has a low carbon equivalent, which reduces the likelihood of cold cracking.
Standardized steel contains a greater amount of alloying elements, which increases its hardenability and reduces the likelihood of cold cracking. It also has a low carbon equivalent.
However, heating hot-rolled steel above 1200℃ can lead to the formation of coarse grain embrittlement, which significantly decreases its toughness.
On the other hand, under the same conditions, the precipitate V in the coarse-grained region of the standard steel is mainly in a solid solution state, leading to a weakening of its ability to inhibit growth and refine the microstructure. This can result in the appearance of coarse grains, higher bainite and MA, leading to a decrease in toughness and an increase in sensitivity to aging.
When planning the welding process, the choice of welding method should be made based on factors such as material structure, plate thickness, required service performance and production conditions.
Low carbon quenched and tempered steel and medium carbon quenched and tempered steel belong to quenched and tempered steel. Are their embrittlement mechanisms in the area affected by welding heat the same?
Why does welding low carbon steel in its quenched and tempered state guarantee good welding quality, while medium carbon steel in the same state generally requires post-weld heat treatment?
Answer: Low Carbon Quenched and Tempered Steel: When subjected to repeated cycles of increasing T8/5, low carbon quenched and tempered steel becomes brittle due to the thickening of austenite and the formation of higher bainite and MA constituents.
Quenched and tempered medium carbon steel: This type of steel has a high carbon content and several alloying elements, which results in a strong hardening tendency, low martensitic transformation temperature and absence of a self-tempering process.
As a result, welding in the heat-affected zone can cause a significant amount of M-structure formation and potential brittleness.
In contrast, low carbon quenched and tempered steel typically benefits from a moderate to low heat input during welding, while the best results for medium carbon steel are achieved through the use of a high heat input during welding. and immediate post-welding heat treatment.
What is the difference between the weldability characteristics of Pearlite heat resistant steel and low carbon quenched and tempered steel?
What is the difference between the selection principle of welding materials for Pearlite heat-resistant steel and resistive steel? why?
Answer: Cold cracking can occur in both pearlite heat-resistant steel and low-carbon quenched and tempered steel.
The heat-affected zone and reheat cracks may experience hardening and embrittlement during heat treatment or prolonged use at high temperatures.
However, in quenched and tempered steels with low carbon content, hot cracking may occur in steels with high nickel content and low manganese content. Furthermore, improper material selection can cause heat-resistant pearlitic steel to crack when hot.
When selecting heat-resistant pearlitic steel, it is important to consider not only the strength of the material, but also the principles of using the joint at high temperatures.
It is also crucial to ensure that welding materials are dry, as Heat Resistant Pearlitic Steel is used at high temperatures and must meet certain strength requirements.
Welding of stainless steel and heat-resistant steel
- Stainless Steel: Refers to steel used in atmospheric environments and aggressive chemical media.
- Heat-resistant steel: Includes oxidation-resistant steel and high-temperature resistant steel. Oxidation-resistant steel refers to steel that has high-temperature oxidation resistance and has low high-temperature strength requirements.
- High temperature resistant steel: Refers to steel that not only has high temperature oxidation resistance, but also has high temperature resistance.
- Thermal Resistance: Refers to the ability to resist fracture (long-term strength) when subjected to high temperatures over a long period of time or the ability to resist plastic deformation (creep resistance) when subjected to high temperatures over a long period of time period of time.
Some concepts:
Chromium equivalent: The relationship between the composition and structure of stainless steel is represented in a diagram. The elements that form ferrite are transformed into a sum of chromium (Cr) elements, taking into account their level of influence. This sum is called Chromium Equivalent, with a coefficient of 1 for chromium.
Nickel equivalent: In the same diagram, the elements that form austenite are transformed into a sum of nickel elements (Ni), considering their level of influence. This sum is called Nickel Equivalent, with a coefficient of 1 for nickel.
Embrittlement at 4750°C: This form of embrittlement occurs when ferritic stainless steel with a high chromium content is heated for a prolonged period at temperatures between 400°C and 540°C. It is called 4750°C brittleness because its most sensitive temperature is around 475°C. At this temperature, the strength and hardness of the steel increase, while its plasticity and toughness decrease significantly.
Solidification Mode: The solidification process begins with crystallization, followed by the completion of the process with the γ or δ phase.
Stress corrosion cracking: Refers to cracks that form in a weak corrosive medium below the yield point of the material, under the combined action of stress and the corrosive medium.
Embrittlement of the σ Phase: The σ phase is a brittle, hard, non-magnetic intermetallic composite phase with a complex compositional crystalline structure.
Intergranular Corrosion: This refers to selective corrosion near grain boundaries.
Chromium deficiency mechanism: The supersaturated solid carbon solution diffuses to the grain boundaries, forming chromium carbide (Cr23C16 or (Fe, Cr)C6) with chromium near the boundary and precipitating at the grain boundary. Because carbon diffuses much faster than chromium, it is too late for chromium to be supplemented from within the crystal to near the grain boundary, resulting in a mass fraction of Cr in the layer adjacent to the grain boundary that is lower. to 12%, which is referred to as “chromium deficiency”.
