What problems are prone to occur when welding medium carbon quenched and tempered steel?

Medium carbon quenched and tempered steel is by adding high carbon and more alloying elements to ensure hardenability, mainly using martensitic phase transformation strengthening to achieve high strength and high hardness, so its toughness is poor, so the weldability is poor, which can also be derived from the calculation of carbon equivalent. The cold cracking tendency of the steel is very obvious, one is due to the hardening tendency is very obvious, and the other is due to the low MS point, it is difficult to produce self-tempering. When welding, in addition to preheating before welding, timely tempering must be carried out after welding.

The usual preheating temperature should be 200, the tempering temperature should be carried out according to the needs of its performance, the higher the temperature, the better the toughness, the lower the strength, and at the same time pay attention to avoid tempering brittleness.

Another significant feature of quenched and tempered steel during welding is the softening of HAZ, at the AC1 tempering temperature, the dislocation movement inside the material will occur, and the further release of distortion stress will reduce the strength. The embrittlement of the coarse grain zone is also one of the weaknesses of HAZ, the heat input is large, although the cooling time will increase, but due to the high hardening tendency of medium carbon quenched and tempered steel, the coarse grain zone of HAZ will still be martensite structure when the cooling time is increased, and the large heat input grain growth is intensified, which makes the embrittlement more serious. Therefore, it is advisable to use a small heat input + preheating.

At the same time, the small heat input also helps to reduce the width of the softening zone. The layer temperature should not be lower than the preheating temperature.

The state of medium carbon quenched and tempered steel before welding is very important, and when welding in the annealed state, the overall quenching and tempering after welding meets the performance requirements. The starting point of determining the process parameters is mainly to ensure that there are no cracks before the quenching and tempering treatment, and the performance of the welded joint is guaranteed by the post-welding heat treatment.

In addition to cracks, the embrittlement of the coarse grain area of HAZ and the softening of the high-temperature tempering area are all sudden during welding in the quenched and tempered condition. Among them, the hardening and embrittlement caused by high carbon M can be solved by tempering after welding, but the softening problem of the high-temperature tempering area cannot be saved by quenching and tempering after welding. Therefore, when determining the welding process parameters in the quenched and tempered state, the main thing should be to prevent cold cracks and avoid softening.

Compared with low carbon steel, the weldability of medium carbon steel is slightly poor, and the hardened structure of low plasticity is easy to produce in the near seam area of the base metal, so when medium carbon steel is welded, the welding consumables and process parameters are improper, and it is easy to produce cold and hot cracks. In addition, due to the high amount of carbon, the stomatal sensitivity is slightly larger.

The carbon content and alloying element content of medium carbon quenched and tempered steel are high, the liquid-solid interval is large, the hardening tendency is obvious, and welding cracks are prone to occur. In addition to the weld crack, the high-carbon martensitic in the heat-affected zone of the weld causes hardening and embrittlement, and the strength is reduced due to high-temperature tempering and softening.

If low-carbon alloy steel and quenched and tempered steel can not be welded together, quenched and tempered steel is also divided into low-carbon quenched and tempered steel, and medium-carbon quenched and tempered steel is also divided into tempered and quenched and tempered welding, and low-alloy steel is due to the high content of alloying elements

Hot cracks, cold cracks, embrittlement and softening of the heat-affected zone.

Brittlement and softening of welding heat-affected zone - First, due to the high carbon content and many alloying elements of medium-carbon quenched and tempered steel, the hardening tendency of steel is large, and a large amount of martensite will be produced in the quenching zone of heat-affected zone, resulting in severe embrittlement. Secondly, the heat-affected zone is heated to an area that exceeds the tempering temperature during quenching and tempering, and there will be a softening zone with lower strength and hardness than the base metal.

Severe crack tendency - the hardening tendency of medium carbon quenched and tempered steel is large, and the martensitic structure generated in the heat-affected zone increases the cold cracking tendency of welded joints. In addition, the medium carbon quenched and tempered steel has a high content of carbon and alloying elements, a large crystallization temperature range of the molten pool, and serious segregation, so it has a large thermal crack susceptibility.

The main problems are cracks (mainly cold cracks), embrittlement and softening problemsLow-carbon quenched and tempered steel plus the hood cavity rolls into the very close elements to improve the hardenability, the hardenability is improved, when M changes the cooling rate of the material residue is faster, can not get the "self-tempering" effect, so that the cold crack tendency increases, in order to strengthen its hardenability and improve the stability of anti-tempering, add alloying elements, most of these elements can cause reheat cracks.

b.Embrittlement at the overheated place: Some reasons that are not conducive to the formation of low carbon M and B will cause the plasticity and toughness to decrease, and the brittleness to increase.

softening; As a result of the accumulation of carbides, the round silver will soften.

Summary. Hello dear, high-strength medium carbon quenched and tempered steel, with a certain plasticity, toughness and strength, good machinability, good comprehensive mechanical properties after quenching and tempering treatment, poor hardenability, easy to crack, welding performance is not high, need to be well preheated before welding, heat treatment is required after welding. Medium carbon steel is mainly used to manufacture high-strength moving parts, such as air compressors, pump pistons, steam turbine impellers, heavy machinery shafts, worms, gears, etc., surface wear-resistant parts, crankshafts, machine tool spindles, rollers, fitter tools, etc.

Hello dear, high-strength medium carbon quenched and tempered steel, with a certain plasticity, toughness and strength, good machinability, good comprehensive mechanical properties after quenching and tempering treatment, poor hardenability, easy to crack, welding performance is not high, welding needs to be well preheated before welding, and heat treatment is required after welding. Medium carbon steel is mainly used to manufacture moving parts with high strength orange content, such as pistons of air compressors and pumps, impellers of steam turbines, shafts, worms, gears, etc. of heavy-duty machinery, wear-resistant parts on the surface, crankshafts, machine tool spindles, rollers, fitter tools, etc.

Hello pro No. 45 steel is a medium carbon steel, often used in the manufacture of mechanical parts, mostly repairing welding, so the weldability is poor, if you still weld according to the commonly used process of welding low carbon steel, often welding land parts will produce cracks and lead to product scrap. When welding No. 45 steel, if the selection of welding materials and the control of the welding process are not good, the weld and near seam area may produce a hard and brittle martensitic structure, resulting in the deterioration of the joint performance, which is easy to damage under vibration or fatigue load, and is also the main factor that induces cold cracks. Additional Information:

No. 45 steel cold plasticity is general, annealing, normalizing is slightly better than quenching and tempering, with higher strength and good machinability, after appropriate heat treatment can obtain a certain toughness, plasticity and wear resistance, the material is convenient. It is suitable for hydrogen welding and argon arc welding, and is not very suitable for gas welding. Preheating is required before welding, and stress relief annealing should be carried out after welding.

Normalizing can improve the cutting performance of high die blanks made of wool with a hardness of less than 160HBS. After quenching and tempering, the comprehensive mechanical properties of the steel should be optimized to be pure and slow compared with other medium-carbon structural steels, but the hardenability of the steel is low, and the critical quenching diameter in water is 12 17mm, and there is a tendency to crack when water quenching.

High-strength medium carbon quenched and tempered steel, with a certain plasticity, toughness and punch strength, good machinability, quenched and tempered silver paddle has good comprehensive mechanical properties, poor hardenability, easy to crack, welding performance is not high, it needs to be well preheated before welding, and heat treatment is required after welding. No. steel is medium carbon steel, often used in the manufacture of mechanical bulk parts, mostly repairing welding, so the weldability is poor, if it is still welded at will according to the commonly used process of welding low carbon steel, often the weldment will produce cracks and lead to product scrapping.

1. Welding method Welding low-carbon quenched and tempered steel, electrode arc welding, submerged arc welding, gas shielded welding, electroslag welding, etc. can be used. However, for S 686MPa grade steel, the most suitable method is gas shielded welding; For steels of the S 980MPa grade, tungsten TIG welding or vacuum electron beam welding should be used.

2. Welding materials When the base metal is welded in the quenched and tempered state, because this kind of steel is generally no longer quenched and tempered after welding, the welding material selected should make the weld metal in the welding state have the same mechanical properties as the base metal; When the base metal is welded in the annealed (or normalized) state, the selected welding material should ensure that it has the same mechanical properties as the base metal after the weldment is quenched and tempered after welding, that is, the welding material with similar chemical composition to the base metal should be selected. In addition, special attention should be paid to the low or ultra-low hydrogen properties of the welding consumables.

3. Welding heat input starts from preventing cold cracks, and requires a slower cooling speed, and from preventing embrittlement in the superheated zone and reducing the softening zone, it requires a faster cooling speed, so the selection of heat input should try to make the cooling rate in the range of neither cold cracks nor embrittlement in the superheated zone. The general practice is to maximize the heat input on the premise of satisfying the toughness of the heat-affected zone. But there is a question of the maximum allowable value here, Table 29 and Table 30 list the maximum allowable value for heat input for T-1 steel and HY-100 steel, respectively.

If the maximum heat input is used, there is still a tendency to crack cold, and preheating or postheating measures should be taken.

4. Preheating temperature The purpose of preheating is not to eliminate low-carbon martensitic structure, but to reduce the cooling rate of martensite transformation and create martensitic "self-tempering" conditions, so as to avoid cold cracks, therefore, the preheating temperature should not be too high. Otherwise, it will increase the brittleness of the superheated zone. If possible, a lower temperature preheat plus post-heat, or no preheating and only post-heat, will give better results.

The minimum preheating temperature and interlayer temperature of several low-carbon quenched and tempered steels are shown in Table 31.

5. Post-welding heat treatment Under normal circumstances, the product is no longer heat treated after welding. However, if the toughness of the steel after welding or cold working is too low, the structural dimension is required to be stable or the weldment is required to be resistant to stress corrosion, post-weld heat treatment is required. The temperature of the stress-relieving heat treatment should avoid the reheat crack sensitive temperature.

In order to ensure the strength of the material, the stress-relieving heat treatment temperature should also be about 30 times lower than the original tempering temperature of the steel.

Summary. Hello <>

The function of preheating and post-welding heat treatment in the welding process of low-carbon quenched and tempered steel is to ensure the quality and performance of welded joints. Preheating can increase the temperature of the welding area, so that the stress in the welding process can be relieved, and at the same time, it can also improve the toughness and strength of the welded joint, and reduce the occurrence of cracks. Post-weld heat treatment can further improve the performance of welded joints, making them have better mechanical properties and corrosion resistance.

2.What is the role of preheating and post-welding heat treatment in the welding process of low-carbon quenched and tempered steel?

Hello <>

The role of preheating and post-welding heat treatment in the welding process of low-carbon quenched and tempered steel is to ensure the quality and performance of the welded joints in the suffocation shed. Preheating can increase the temperature of the welding area, so that the stress in the welding process can be relieved, and at the same time, it can also improve the toughness and strength of the welded joint, and reduce the occurrence of cracks. The hot and noisy old treatment can further improve the performance of the welded joint, so that it has better mechanical properties and corrosion resistance.

The choice of preheating temperature and time should be determined according to the specific welding material and thickness of the oak, and the preheating temperature should be above 150 degrees for a long enough time to fully eliminate the stress in the welding area. There are many ways of post-weld heat treatment, including normalizing, quenching, tempering, etc., different treatment methods can get different performance, which needs to be selected according to specific use needs. In the welding process, it is also necessary to pay attention to controlling the number of differential rows of welding beams, including welding current, voltage, welding speed, etc., so as to fully ensure the quality and performance of welded joints.

The quality and performance of welded joints are also affected by the welding materials and welding methods, and the appropriate materials and methods need to be selected according to the specific use needs.

The quenched and tempered steel has high strength, cold brittleness, and the high temperature of the weld during welding produces stress in other places, so that the thermal stress decomposes uniformly during welding, it is necessary to have preheating. Pig iron welding too.

Weldability of mild steel.

Due to the low carbon content and low manganese and silicon content of mild steel, it usually does not produce severe hardened or quenched structures due to welding. The plasticity and impact toughness of the joint after low carbon steel welding are good, and when welding, there is generally no need to preheat, control the interlayer temperature and post-heat, and there is no need to use heat treatment to improve the structure after welding, and the whole welding process does not need to take special technological measures, and the weldability is excellent.

But in a few cases, difficulties can also arise when welding:

1. The converter steel produced by the old smelting method has high nitrogen content and high impurity content, resulting in high cold brittleness, increased aging sensitivity, reduced quality of welded joints, and poor weldability.

2. The deoxidation of boiling steel is incomplete, the oxygen content is high, the distribution of impurities such as P is uneven, the content in local areas will exceed the standard, the sensitivity to aging and cold brittleness is large, and the tendency of hot cracks is also increased.

3. The use of welding rods that do not meet the requirements of quality makes the carbon and sulfur content in the weld metal too high, which will lead to cracks. For example, when a factory uses acid electrodes to weld Q235 A steel, the carbon content of ferromanganese in the electrode coating is too high, which will cause thermal cracks in the weld.

4. Some welding methods will reduce the quality of mild steel welded joints. For example, electroslag welding, due to the large line energy, will make the grains in the coarse grain area of the welding heat-affected zone grow very coarse, causing a serious decrease in impact toughness, and it is necessary to normalize the fine grains after welding to improve the impact toughness.

In short, mild steel is the best weldable and easiest type of steel to weld, and all welding methods can be applied to the welding of mild steel.