Adamite steel roll has the advantages of high hardness, good wear resistance and relatively low manufacturing cost. It is widely used in steel rolling industry. However, the repair of adamite steel roll after wear has always been a difficult problem in front of people, which has been paid attention to by the welding industry for many years.
Title：THE BEAD WELDING R EPA IRING TECHNO] LOGY OF THE ADAMITE STEEL ROLLS .
Keyword：adamite steel rolls, bead welding repair
2. Basic condition of adamite steel roll
The chemical composition of adamite steel roll (155crnimo) is as follows: (%) C: 1.5 ~ 1.7, Si: 0.3 ~ 1.0, Mn: 0.7 ~ 1.2, Cr: 1.3 ~ 1.7, Mo: 0.1 ~ 0.6, Ni: 1.3 ~ 1.7
Its basic working conditions are as follows: start rolling temperature: 950 ~ 10509c rolling pressure: 1900 ~ 2200t deformation: 0.5% linear speed: 50 ~ 70m / min (steel passing time is about 100s)
We know that hypereutectoid steel with carbon content of 0.77 ~ 2.11% is composed of secondary cementite and pearlite at room temperature. Due to the loose material of semi steel roll, it is easy to produce pores in welding, and it is easy to exist in the weld when the weld pool crystallizes; This kind of segregation will cause the deterioration of microstructure and properties. In addition, this material also has obvious regional segregation. During welding pool crystallization, due to the continuous growth and movement of columnar crystals, impurities will be pushed to the center of the pool, resulting in higher impurities in the center of the pool. This regional segregation is also one of the reasons for welding cracks.
3 surfacing repair technology
3.1 selection of welding materials
3.1.1 transition layer with reasonable design
As we all know, when the carbon content of steel is ≥ 0.45%, it belongs to the steel with poor weldability. Moreover, the carbon content of semi steel roll is so high that it needs to work continuously under the above working conditions after welding. Therefore, the first thing to be solved is to design a kind of welding material (welding wire and flux) that can be welded with the base metal and a reasonable surfacing process. The welding wire composition of the transition layer shall not only ensure firm fusion with the base metal without cracks, but also have good plasticity and toughness, which is conducive to ensuring the quality of the whole surfacing layer. Through many tests, the ysbd welding wire developed by our factory can meet the above requirements.
3.1.2 design of working layer welding wire: because the transition layer welding material is mainly to improve the combination with the parent, it has good weldability and strong crack resistance. However, its wear resistance is quite different from that of the parent, so a layer of wear-resistant material which can not only meet the requirements of working conditions, but also integrate with the transition layer needs to be overlaid on the transition layer. According to the technological requirements of adamite steel roll, that is, high strength, good wear resistance, good peeling resistance and moderate surface hardness. We select ysc-6 developed by our factory as the working layer welding wire.
3.1.3 the self-developed flux matching with the welding wire shall be used.
3.1.2 welding method
Due to the large amount of surfacing of semi steel roll, the diameter direction is from 856mm to 920mm, and the roll body is 1462mm long. The amount of welding wire is about 1500kg, so we use submerged arc automatic welding for surfacing repair. Submerged arc welding has the advantages of high production efficiency, stable weld quality and sufficient metallurgical reaction, which can ensure the surfacing quality.
3.3 preparation of welding process
3.3.1 pretreatment before surfacing repair
Firstly, the roll body and the joint with the roll neck of the roll to be repaired shall be subject to surface dye detection. If any defect is found, it shall be marked on the physical object, and the coordinate orientation shall be marked on the flaw detection record. Then it shall be sent to the heating furnace for stress relief treatment, and the flaw detection shall be carried out after it is cooled to room temperature. The core shall be measured by ultrasonic detection, and the surface shall be subject to dye detection. Then, the surface fatigue layer shall be removed by machining. If local defects are found, they shall continue to be removed until it is confirmed that there are no defects, and another dye flaw detection shall be conducted for the defects in the original record.
3.3.2 preheating temperature and interlayer temperature
In order to prevent the large strain of the surfacing layer and the martensitic transformation between the base metal and the surfacing layer during the surfacing process, resulting in the cracking of the base metal of the surfacing layer. Therefore, the roll must be preheated before surfacing. During preheating, the roll shall rotate at a uniform speed and heat gradually and slowly to reduce the temperature difference between inside and outside. The preheating temperature shall be determined according to the chemical composition of the roll（ (see Figure 1)
Due to the large roller, fast heat dissipation and long surfacing operation time, in order to avoid the direct transformation of the weld pool from austenite to martensite in the surfacing process and affect the quality of the surfacing layer, the interlayer temperature should be strictly controlled during surfacing.
3.3.3 preparation of welding materials
In order to ensure the smooth surfacing process, the surface of the welding wire shall be degreased and derusted before welding. The tubular welding wire shall be used after drying, and the flux shall be kept at 350 cores for 3 hours.
3.3.4 selection of welding current
When the welding current changes, it has an impact on the fusion width, penetration and reinforcement. With the increase of current, the arc blowing force increases, and the discharge of liquid metal at the bottom of the molten pool increases, so the penetration increases in proportion. Due to the high carbon content and more alloy elements in the base metal of the semi steel roll, in order to prevent the high carbon content of the base metal from affecting the weld quality, we emphasize that the transition layer adopts low current and narrow weld bead to reduce the dilution rate with the base metal.
3.3.5 selection of arc voltage
With the increase of arc voltage, the weld width increases obviously, but the penetration decreases slightly, which weakens the discharge of liquid metal at the bottom of the molten pool. Therefore, we choose a slightly higher arc voltage when designing the arc voltage of the transition layer.
3.3.6 Selection of welding speed
The welding speed increases to a certain value, the weld penetration and weld width decrease with the increase of welding speed, so the welding speed we designed is 30 ~ 32m / h, considering both welding line energy and weld formation. See the following table for basic process parameters:
3.3.7 post weld heat treatment
After surfacing, the roll must be subject to post weld heat treatment. Post weld heat treatment is one of the keys to ensure the surfacing quality. It can eliminate the internal stress of the surfacing layer, improve the microstructure of the welding layer and adjust the hardness of the welding layer, so as to meet the application requirements. The post weld heat treatment shall be carried out in the furnace after the weldment is cooled to room temperature. The heating and cooling shall be carried out smoothly to reduce the temperature gradient in the section thickness direction（ (see Figure 2)
4 quality inspection
(1) Visually inspect the surface quality of the surfacing layer for defects such as pores and cracks.
(2) The quality of surfacing layer shall be inspected by ultrasonic and surface dye detection.
(3) Detect the thickness and hardness of surfacing layer.
5 use effect
The roll has been used for four times, and the amount of steel used each time is 8000 ~ 10000 tons. The roll surface is in good condition, and the wear is equivalent to that of the new roll.
Adamite steel roll has high carbon content, large hardening tendency, strong crack sensitivity and poor welding performance. However, by selecting suitable welding materials, adopting appropriate welding methods, formulating reliable welding process, and strictly controlling a series of technologies such as preheating before welding, interpass temperature and post weld heat treatment, a better repair effect can be achieved. Facts have proved that the semi steel roll repaired by surfacing technology has good effect and user satisfaction after it is put into production and operation.