I. Special requirements for welding pearlite heat-resistant steel

1, typical steel grade and composition:

(1), the total content of gold is generally less than 5% ~ 7%, and the pearlite structure obtained after normalizing has good thermal strength at 500℃ ~ 600℃, and the cold working, hot working and welding properties are also good, and the price is relatively cheap. Therefore, this kind of steel is widely used to manufacture steam power generation equipment, among which Cr-Mo steel and Cr-Mo-V steel are the most widely used. The content of Cr in this kind of steel is generally 0.5% ~ 9%, and the content of Mo is generally 0.5% ~ 1%. With the increase of Cr and Mo content, the high temperature strength, oxidation resistance and sulfide corrosion resistance of this kind of steel are also improved. In addition, a small amount of alloying elements such as V, W, Ti and Nb are added. The thermal strength of this steel can be further improved. Commonly used pearlite heat-resistant steel and its chemical composition are shown in table.

(2) The main weldability problems of pearlite heat-resistant steel.

Similar to low-carbon quenched and tempered high-strength steel, HAZ has hardening, cold cracking, softening, reheat cracking and tempering embrittlement.

1), cold crack of welded joint:

One of the most common welding defects of pearlite heat-resistant steel is the cold crack in the coarse grain zone of heat affected zone. In practical production, in order to prevent cold cracking, measures such as preheating before welding, controlling interlayer temperature, dehydrogenation after welding, improving microstructure, reducing and eliminating stress are generally adopted.

2), hot cracks in the weld:

Pearlite heat-resistant steel used in practical production rarely produces thermal cracks in the heat affected zone, but mostly produces cattle in the weld, especially in the arc pit. The occurrence of hot cracks is directly related to the solidification temperature range of pearlite heat-resistant steel. The larger the solidification temperature range, the greater the tendency of hot cracking; On the contrary, the tendency of thermal cracking is small. This kind of thermal crack usually occurs at the junction of columnar crystals. The junction of columnar crystals is often the last solidification position of liquid metal in weld, and it is also the enrichment position of impurities and low melting point crystals. The results show that alloying elements S, C, P, Cu and Ni can promote the generation of hot cracks, while Mn and Ca can inhibit the generation of hot cracks to some extent.

3) Reheat crack in heat affected zone:

Adding a small amount of alloying elements such as Cr, Mo, V, Ti and Nb to this steel, which are all strong carbide forming elements, will increase the reheat crack sensitivity of the steel. Reheat cracks usually appear in the thick part of the workpiece. Therefore, in the welding process of thick plate structure, after the weld is welded to a certain thickness, the intermediate stress relief heat treatment is carried out first, which is beneficial to prevent reheat cracks.

4), temper embrittlement phenomenon:

Tempering embrittlement of Cr-Mn steel is mainly caused by segregation of impurity elements P, As, Sn and Sb on grain boundaries after long-term heating in the range of temper embrittlement temperature, and it is also related to elements Mn and Si that promote temper embrittlement. Therefore, for Kitsiou metal, strictly controlling the content of harmful impurity elements and reducing the content of Mn and Si are effective measures to solve embrittlement.

Second, the welding process characteristics of pearlescent heat-resistant steel

1, heating and interlayer temperature control:

Preheating and interlayer temperature control are effective technological measures to prevent cold cracks in pearlite heat-resistant steel. In general, the preheating temperature and interlayer temperature of pearlite heat-resistant steel should be controlled between 150 ~ 350℃.

2, welding method:

The two most common welding methods in welding production are TIG welding, manual arc welding and automatic submerged arc welding.

3. Tempering after welding:

Pearlite heat-resistant steel is generally supplied after heat treatment, and most of these steels need high temperature tempering after welding.

4. Selection of welding materials:

Pearlite heat-resistant steel works under high temperature and pressure for a long time, which requires high quality of joints. No matter the mechanical properties at room temperature, high temperature, oxidation resistance or organizational stability, it must meet the technical requirements of product operation. Therefore, the selection of welding materials is very important. The selection of welding materials should strive to make the weld metal composition and mechanical properties match the base metal. If the composition of the weld metal is quite different from that of the base metal, the joint will work at high temperature for a long time or undergo post-weld heat treatment, and some elements will diffuse due to uneven composition, resulting in a significant decline in the durability of the joint. However, in actual welding production, in order to prevent weld metal from hot cracking, the carbon content of weld metal is generally lower than that of base metal (but generally not lower than 0.07%), and the performance of weld metal is sometimes lower than that of base metal. However, if the welding material is selected properly, the performance of weld metal can completely match that of base metal. In addition, in the case of welding defects or post-weld heat treatment, covered electrode of austenitic steel can also be selected to prevent cold cracking. However, working at high temperature for a long time will lead to phase brittleness of weld metal.

Three. Welding technology of pearlite heat-resistant steel

Steel with sufficient strength and oxidation resistance at high temperature is called heat-resistant steel, which is a low-alloy heat-resistant steel with Cr and Mo as the main alloying elements and pearlite (or pearlite+ferrite) as the matrix structure. Commonly used steel grades are 15CrMo, 12CrMoV, 12Cr2MoWVTiB and 14MnMov.

Because pearlite heat-resistant steel contains a certain amount of alloying elements such as Cr and Mo, hard and brittle martensite structure will be produced in the heat affected zone, and cold cracks are easy to form when welding rigid structures at low temperature. Therefore, the following technical measures should be taken during welding:

(1) Preheating is an important process measure for welding pearlite heat-resistant steel. In order to ensure the welding quality, the weldment should be preheated and kept at 80 ~ 150℃ during tack welding or formal welding. When argon arc welding is used for backing and CO2 gas shielded welding, the preheating temperature can be reduced or not.

(2) After slow cooling after welding, immediately cover the weld and heat affected zone with asbestos cloth to make it slow cooling.

(3) Post-weld heat treatment After welding, high-temperature tempering should be carried out immediately to prevent delayed cracks, eliminate stress and improve the structure. The post-weld heat treatment temperature should be avoided in the temperature range of 350 ~ 500℃, because pearlite heat-resistant steel has strong brittleness in this temperature range.

4. See table 1 1 for the post-weld heat treatment temperatures of several commonly used pearlite heat-resistant steels.

Table 1 1 Post-weld Heat Treatment Temperature of Pearlite Heat Resistant Steel

Thickness of steel to be heat treated (mm) High temperature tempering temperature after welding (℃)

15CrMo >6 680~700

12cr 1 mov > 10 720 ~ 760

20 crmo 720 ~ 760 Any thickness

12Cr2MoWVB arbitrary thickness 760 ~ 780

12Cr3MoVSiTiB Any thickness is 740 ~ 780.

Five, pearlite heat-resistant steel welding, how to correctly choose welding materials?

The general principle is to select welding materials according to the requirements of chemical composition, that is, the chemical composition of deposited metal should be equivalent to that of base metal.

Selection of welding materials for pearlite heat-resistant steel

Steel grade manual arc welding submerged arc welding gas shielded welding

Covered electrode brand covered electrode welding wire and flux matching welding wire brand.

15 crmo r307e 55 15-B2 h 08 crmoa+iij 350h 08 crmnsimo

12 crmov r 3 17e 55 15-B2-V h 08 crmov+HJ 350h 08 crmnsimov

Cr2Mo R40 e 60 15-B3 h 08 Cr 3 mom na+HJ 350h 08 Cr 3 mom NSI

12 crmowv-TiB R347 e 55 15-B3-V wbh 08 Cr 2 mowvnbb+HJ 250h 08 Cr 2 mnwvnbb

14 mnmov J606 e 60 15-d 1h 08 Mn 2 moa+HJ 350h 08 Mn 2 SIMO

18 mnmo J606 e 60 15-d 1h 08 Mn 2 moa+HJ 350h 08 Mn 2 SIMO

13 mnnimonb j 607 ni E 60 15G h 08 Mn 2 nimo+HJ 350h 08 Mn 2 nimo si

In principle, all kinds of metals can be welded, but the inherent basic properties of metals can not directly indicate what problems will occur during welding and whether the properties of welded joints can meet the requirements of use. Therefore, the adaptability of metal materials to welding process is measured by weldability.

The above is about pearlite heat-resistant steel. I hope you can understand the information after reading it, which will be helpful for your future welding. I wish you success.