Welding Technique of Martensitic Steel: Things You Should Know

The microstructure of martensitic steel (MS Martensitic Steel) is almost all martensitic.

It has high tensile strength, and its maximum strength can reach 1600MPa.

It needs to be tempered to improve its plasticity, so that it can still have sufficient formability under such high strength.

It is the steel with the highest strength level among commercial high-strength steel plates at present.

Welding Technique of Martensitic Steel: Things You Should Know 1

There are two types of martensitic steels:

One is simple Cr13 series steel, such as 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc;

The other is a kind of martensitic steel strengthened by multi-element alloy, such as 1Cr11MoV, 1Cr12WMoV, based on Cr12, with W, Mo, V, Ti, Nb and other elements added, to improve the thermal strength.

Martensitic steel has a strong quenching tendency.

Generally, it can be quenched by air cooling of high-temperature austenite to form martensite structure.

However, 1Cr13 with low carbon content has martensite plus ferrite structure after quenching, which belongs to semi martensitic steel.

In the above two types of martensitic steel, the former is mainly used for general corrosion resistance conditions (such as atmosphere, seawater and nitric acid) and components requiring certain strength, and the latter is mainly used for heat strength steel.

Weldability of martensitic steel

Martensitic steels have a great tendency to harden.

Under the condition of air cooling, martensite with high hardness can be produced, and its weldability is the worst among all stainless steels and high alloy heat-resistant steels.

The following problems are easy to occur during welding:

1. Welding cold crack

This is a very prominent problem of martensitic steel.

On the one hand, it is related to its high hardenability;

On the other hand, it is also related to the poor thermal conductivity of martensite, which can cause large welding internal stress.

In particular, steel with high carbon content and welding structure with high rigidity ratio are easy to produce welding cold cracks.

Therefore, measures such as preheating and post-welding heat treatment are generally required.

2. Embrittlement of welded joint

(1) Overheating embrittlement near the seam

Most martensitic steels are located at the junction of martensite and ferrite due to their composition characteristics.

When the cooling rate is high, coarse martensite can be formed near the joint, which reduces the joint plasticity;

When the cooling rate is low, coarse massive ferrite and carbide structure will be produced, which will significantly reduce the joint shape.

Therefore, the cooling rate should be controlled during welding.

(2) Temper embrittlement

When the martensitic steel and its welded joints are heated and gradually cooled in the range of 375~575 ℃, the fracture toughness can be significantly reduced.

This is caused by temper embrittlement, so the temper embrittlement temperature zone shall be avoided during heat treatment.

Welding Technique of Martensitic Steel: Things You Should Know 2

Key points of the welding process for martensitic steel

1. Welding method

Martensitic steel can be welded by all fusion welding methods except gas welding, such as shielded metal arc welding, submerged arc welding, argon tungsten arc welding, argon metal arc welding, etc.

Because this steel has great cold cracking sensitivity, the weldment must be strictly cleaned and the welding rod must be dried before welding, so that the welding can maintain low hydrogen or even ultra-low hydrogen conditions.

When the constraint degree of the welded joint is large, it is better to adopt argon tungsten arc welding or argon melt arc welding.

The tendency to produce cold cracks can be reduced by appropriately increasing the welding heat input without overheating and embrittlement near the weld.

2. Welding materials

The selection of welding materials shall be subject to different steel grades, welding methods and working conditions of joints.

In order to ensure the requirements of service performance, the chemical composition of the weld shall be close to that of the base metal, that is, the welding materials close to that of the base metal shall be selected.

But in this case, the weld and heat-affected zone are easy to harden and become brittle.

In order to prevent cold cracking, heat treatment is generally required after welding.

When heat treatment is not allowed for weldments, 25-20 and 25-13 type austenitic steel welding materials should be used for welding to form austenitic welds, relax welding stress, and reduce cold cracking tendency by more hydrogen solution.

Austenitic welds have high plasticity and toughness, but low strength, so they are only suitable for weldments working under static load conditions with low stress.

Moreover, due to the large difference in the thermophysical properties between the weld and the base metal, when working at high temperatures, higher additional stress can be generated at the interface of the joint and lead to early failure of the joint, so they are not suitable for weldments working at high temperatures.

Low hydrogen electrode is usually used for arc welding with welding rod, and it shall be dried at 400~450 ℃ for two hours before welding.

Submerged arc welding shall use low silicon high alkaline or weak acid flux, such as HJ172, HJ173, HJ251, etc.

TIG welding is mainly used for backing welding and thin piece welding in multi-layer welding.

3. Preheating and interpass temperature

Preheating and maintaining the interpass temperature is an important technological measure to prevent cold cracks.

The selection of preheating temperature should first consider the carbon content in the steel, and then consider the restraint degree of the joint, the composition of filler metal and welding method.

Table 1 shows the recommended preheating temperature, heat input, etc. according to the classification of carbon content.

If the binding degree of the joint is large, the preheating temperature and interpass temperature shall be increased accordingly.

The interpass temperature shall not be lower than the preheating temperature.

When welding with austenitic steel welding materials, depending on the thickness of the weldment, preheating or low temperature preheating may not be required.

Table 1 Recommended Preheating Temperature and Heat Input for Martensitic Steel Welding

Mass fraction of carbon (%Preheating temperature range/℃Welding heat inputPost weld heat treatment requirements
Below 0.10100-150Medium heat inputBy wall thickness
0.10~0.20150~250Moderate heat inputHeat treatment is required for any thickness
0.20-0.50250~300High heat inputHeat treatment is required for any thickness

4. Post-weld heat treatment

Post-weld heat treatment is another important process measure to prevent cold cracks.

When selecting welding materials with similar composition to the base metal, post-weld tempering heat treatment is generally required.

When austenitic steel welding materials are selected for welding, post-weld heat treatment is generally not required.

In order to ensure that the austenite can be completely transformed into martensite after welding, it is not allowed to conduct tempering treatment immediately after welding.

The joint must be cooled to a temperature below Ms point and maintained for a certain time before high-temperature tempering treatment.

Because if tempered immediately after welding, the austenite will be transformed into pearlite and the carbide will precipitate along the austenite grain boundary, which is very brittle.

However, in order to prevent cold cracking, it is not allowed to conduct high-temperature tempering treatment after the joint is cooled to room temperature.

Generally, tempering treatment is conducted when the joint is cooled to 100~150 ℃.

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