The microstructure of Martensitic Steel (MS) is predominantly martensitic. It has high tensile strength, with maximum strength reaching 1600 MPa. To increase its plasticity, the steel must be tempered, allowing it to maintain sufficient formability despite its high resistance.
Currently, Martensitic Steel has the highest level of resistance among commercially available high-strength steel plates.
Martensitic steel is categorized into two types:
- Cr13 series plain steel, including 1Cr13, 2Cr13, 3Cr13, 4Cr13 and so on.
- Martensitic steel reinforced with multi-element alloy such as 1Cr11MoV and 1Cr12WMoV, which is based on Cr12 and includes elements such as W, Mo, V, Ti, Nb and others to increase its thermal resistance.
Martensitic steel is known for its strong tempering tendency, which can be achieved by air cooling high-temperature austenite to form a martensite structure. However, low-carbon 1Cr13 forms a semi-martensitic structure with martensite and ferrite after quenching.
The first type of martensitic steel is mainly used in general corrosion resistance conditions, such as in atmospheres, sea water and nitric acid, as well as in components that require a certain level of resistance. The latter type is mainly used for heat-resistant steel.
Weldability of martensitic steel
Martensitic steels have a strong tendency to harden. When air cooled, high hardness martensite can be produced. However, this also leads to the worst weldability among all stainless steels and heat-resistant high-alloy steels.
The following problems are commonly encountered during welding:
1. Cold crack welding
This is a well-known problem with martensitic steel.
On the one hand, it is due to its high hardenability. On the other hand, it is also a result of martensite's low thermal conductivity, which can lead to significant internal stresses during welding.
In particular, high-carbon martensitic steel and rigid welding structures are prone to developing cold cracks during welding.
To resolve this, measures such as preheating and post-weld heat treatment are often necessary.
2. Welded joint weakening
(1) Embrittlement due to overheating near the seam
Martensitic steels are often located on the border of martensite and ferrite due to their compositional characteristics.
When the cooling rate is high, large martensite grains can form near the joint, reducing its plasticity.
If the cooling rate is low, a coarse structure of solid ferrite and carbides will form, which significantly diminishes the shape of the joint.
Therefore, it is essential to control the cooling rate during welding.
(2) Weakness due to Temperament
Martensitic steels and their welded joints can be susceptible to temper embrittlement, which can significantly reduce their fracture toughness, when heated and cooled slowly in the temperature range of 375 to 575°C.
Therefore, it is crucial to avoid this temperature range during heat treatment to avoid temper embrittlement.
Key points of the martensitic steel welding process
1. Welding method
Martensitic steel can be welded using all fusion welding methods except gas welding, including shielded metal arc welding, submerged arc welding, argon tungsten arc welding, and argon metal arc welding, among others. others.
However, due to its high sensitivity to cold cracking, it is important to thoroughly clean the weldment and dry the welding rod before welding to ensure low or even ultra-low hydrogen conditions.
When the joint restriction degree is high, it is recommended to use argon tungsten arc welding or argon metal arc welding.
To minimize the risk of cold cracks, it is important to increase the welding heat input appropriately, avoiding overheating and embrittlement in the vicinity of the weld.
2. Welding materials
The choice of welding materials should be based on the type of steel, the welding method and the working conditions of the joint.
To ensure the joint performs as desired, it is important to choose welding materials that have a chemical composition close to that of the base metal. However, this can cause the weld and heat affected zone to harden and become brittle.
Heat treatment is often necessary after welding to prevent cold cracking. When heat treatment is not possible, 25-20 and 25-13 type austenitic steel welding materials can be used to form austenitic welds, which can alleviate welding stress and reduce the tendency of cold cracking due to to the increase in hydrogen content.
Austenitic welds have good plasticity and toughness, but low strength, making them only suitable for joints under low stress static loading conditions. Furthermore, the large difference in thermophysical properties between the weld and the base metal can result in additional stress at the joint interface when working at high temperatures, leading to early joint failure, therefore they are not suitable for high temperature applications.
Low hydrogen electrodes are typically used for arc welding with welding rods and must be dried at 400-450°C for two hours before welding. Submerged arc welding should use high alkaline flux or weak acid with low silicon content, such as HJ172, HJ173 or HJ251. TIG welding is mainly used for backing welding and welding of thin parts in multilayer welding.
3. Preheating and temperature between passes
Preheating and maintaining temperature between passes are crucial steps to prevent cold cracking during welding.
The preheating temperature should be determined based on the carbon content of the steel and then taking into account the degree of joint restraint, the filler metal composition and the welding method. Table 1 provides recommended preheat temperatures, heat inputs, etc. based on carbon content classification.
If the joint has a high degree of restriction, it is necessary to increase the preheating temperature and interpass temperature accordingly. The temperature between passes must not be lower than the preheating temperature.
For welding with austenitic steel welding materials, preheating or low temperature preheating may not be necessary depending on the thickness of the weldment.
Table 1 Recommended preheating temperature and heat input for welding martensitic steel
| Mass fraction of carbon (%) | Preheating temperature range/℃ | Welding heat input | Post-weld heat treatment requirements |
| Below 0.10 | 100-150 | Average heat input | By wall thickness |
| 0.10~0.20 | 150~250 | Moderate heat input | Heat treatment is required for any thickness |
| 0.20-0.50 | 250~300 | High heat input | Heat treatment is required for any thickness |
4. Post-welding heat treatment
Post-welding heat treatment is another important measure to prevent cold cracking during welding.
When welding materials with a composition similar to that of the base metal are used, a post-weld quenching heat treatment is normally required. On the other hand, when welding with austenitic steel welding materials, post-weld heat treatment is generally not necessary.
To ensure the complete transformation of austenite to martensite after welding, it is important to avoid tempering treatment immediately after welding. The joint must be cooled to a temperature below the Ms point and maintained at that temperature for a specified time before being subjected to high-temperature tempering treatment. This is because if tempering is done immediately after welding, the austenite will transform into pearlite and the carbides will precipitate along the austenite grain boundary, making the joint very brittle.
However, to avoid cold cracking, high-temperature tempering treatment should not be carried out after the joint has cooled to room temperature. Typically, tempering treatment is carried out when the joint cools to 100-150°C.
