Research on surfacing repair technology of back-up rolls in hot strip rolling mill

Abstract: By analyzing the base material of the roll, select the appropriate surfacing material. Develop a special fixture for back-up roll surfacing welding and determine the type of submerged arc welding machine. Through the production and testing of simulated rollers, the cladding process and heat treatment process are determined and optimized. The overlay welding repair of the two support rollers was completed in stages.

Keywords: backup roll; surfacing process; wear resistance

In recent years, with the increasing maturity of surfacing technology, the use of surfacing method to repair some types of old rolls to extend the service life of the rolls has become an important measure for my country’s large and medium-sized enterprises to reduce costs and improve equipment operation rates. It is also in line with my country’s energy conservation and consumption reduction. , green environmental protection requirements. Surfacing technology has also become an economical, fast and effective means of manufacturing and maintenance in the machinery manufacturing industry. Baosteel Co., Ltd. has three hot rolling lines that use forged or cast backup rolls from professional roll manufacturers. Typical chemical composition is: 0.4%~0.5% C, 5% Cr, and a small amount of other alloying elements. The typical metallographic structure of the working layer is martensite or lower bainite and a small amount of Cr₇C₃ carbide, and the typical metallographic structure of the roll neck is pearlite. The allowable hardness range of the new backup roll is 65 to 71HSD, the roller surface hardness fluctuation is less than 2HSD, and the roll neck hardness is 35 to 45HSD. At present, Baosteel consumes more than 10 hot-rolled backup rolls every year, and it is very necessary to carry out surfacing repair on their surfaces.

1 Design of back-up roll surfacing layer

1.1 Selection and manufacturing of welding materials for cladding layer of hot-rolled backup rolls

When selecting welding materials, the following indicators should be considered:

(1) The compatibility of the roll body and the welding material means that the two must be mutually soluble to avoid the generation of brittle metal compounds. It is also necessary to consider the differences in physical properties such as melting temperature, expansion coefficient, thermal conductivity and specific resistance between the two.

(2) The composition, properties and structure of the fusion zone. The fusion zone is the boundary area between the surfacing layer and the substrate and the boundary area between the surfacing layers. The boundary zone generally includes the fusion line and the crystallization transition zone. Since the crystallization characteristics of each layer in the welding zone are different, a transition layer with poor performance may be formed due to changes in composition. For example, when an alloy steel welding layer is cladded on low carbon steel, the C content in the transition layer increases greatly, forming a large amount of brittle metal compounds, causing cracking of the welding layer. During the surfacing process, the generation and growth of the transition layer should be avoided or controlled as much as possible, and elements and processes that can inhibit the generation of brittle phases should be used as much as possible, such as surfacing materials containing a large amount of austenite-forming elements. Based on the above principles, the selected welding material composition is shown in Table 1.

Table 1 Chemical composition of welding materials (mass fraction, %)

 CCrSiMoMnNiFluxHRC
base layer0.101.000.800.601.90.9HJ26028
transition layer0.352.660.23 1.640.85HJ26032
working layer0.265.280.323.432.513.4HJ26045

1.2 Preparation before welding wire and flux surfacing

Submerged arc automatic welding has been widely promoted in manufacturing processes such as metal structural parts due to its high efficiency and stable quality. However, when applied to the welding production of medium and low carbon steel and low alloy structural parts, the most prominent problem during the welding process of such steel structures is the occurrence of cracks in the welds of the workpiece. One of the main reasons is the moisture content in the welding materials. Therefore, when using the submerged arc welding process to surfacing large support rollers, the flux must be strictly dried.

1.3 Design of surfacing layer

The mechanical property requirements of surfacing materials are shown in Table 2.

Table 2 Mechanical properties of hard surface (working) layer materials

Rp0.2/MPaRm/MPaA(%)KV/JHSD
885~9001150~11602.5~4.518~1958~62

(1) Base layer: Use low-carbon and low-alloy surfacing materials to ensure good compatibility between the welding material and the base metal and sufficient plasticity to ensure the welding strength of the welding material and the base metal. Large-scale forged support rollers are all high-carbon alloy steel. The carbon equivalent of the base material is high and the weldability is poor. In order to ensure a good metallurgical bond between the base metal and the cladding metal, the base layer must be cladded before the hardfacing working layer. The base layer material should be selected according to the alloy composition of the base metal to match the low and medium carbon surfacing alloy system. It is necessary to consider the metallographic structure of the alloy system and the strength and toughness of the surfacing material. Both the roller body and the hard surface working layer have wettability and good bonding properties. When the alloy composition is slightly lower, a stainless steel alloy system with better toughness should be selected. Micro-alloying elements are added to the cladding alloy system to refine the grains and change the shape of inclusions, thereby improving the strength and toughness of the cladding alloy.

(2) Transition layer: The hardness and strength of the surfacing layer of the transition layer are between those of the base layer and the working layer. It has good compatibility with the two and serves as a link between the past and the next, laying a good foundation for the surfacing hard surface layer. The transition layer is located between the base layer and the working layer and plays a transition buffering role. Therefore, the transition layer material must have good strength and toughness as well as good inter-layer bonding performance. The transition layer generally has a surfacing thickness of 7 to 14 mm, and the surfacing layer structure is selected from bainite + sorbite + a small amount of other structures and carbides or bainite + martensite + a small amount of other structures and carbides.

(3) Working layer: Use welding wire with higher alloy content to improve the strength and wear resistance of the working layer. At the same time, the working layer welding material should also have corresponding corrosion resistance to ensure the performance of the roll. The mechanical properties are shown in Table 2.

2 Back-up roll surface surfacing treatment

2.1 Surfacing process design

Adopting reasonable surfacing process parameters is an effective means to ensure a stable surfacing process and good weld quality. The main process parameters that affect the quality of surfacing welding are:. Welding voltage, current and welding speed, rotation speed of the workpiece, thickness of the welding layer, overlap amount between weld beads, welding position, gradient of the surfacing part of the workpiece, corner curvature of the step, etc.

2.1.1 Turning

Before cladding, ultrasonic testing and coloring testing are used to determine the location and depth of defects, including fatigue layer turning of the old roll ring and roll neck. This is the key to ensuring the quality of the final cladding of the backup roll.

2.1.2 Preheating

The main purpose of preheating is to reduce the cooling rate of the cladding metal and the heat-affected zone during the cladding process, reduce the hardening tendency and reduce the welding stress, and prevent the base metal and cladding metal from phase change during the cladding process, resulting in cracks. The determination of the preheating temperature needs to be based on the C content and alloy content of the base metal and surfacing material, and should be higher than the martensitic transformation temperature Ms point of the material.

As far as the back-up roller is concerned, since the alloy content of the base material and the cladding material is relatively high, coupled with the large size and cladding thickness of the back-up roll, the welding stress is large. In order to prevent cracks and pores, improve the fusion of the weld layer and the base metal when starting surfacing, and avoid cracks and welding defects, the preheating temperature should be increased as much as possible. Taking into account the allowable degree in actual operation, stress relief should be maintained at 560°C for 70 hours before surfacing, and the furnace should be cooled to 370°C to start surfacing. During the heating process, in order to ensure that the inside of the roller body is heated through and the temperature inside and outside the roller body is uniform, the temperature rise should be slow, and the preheating temperature rise rate should be ≤25°C/h. The preheating and holding time is determined according to the outer diameter of the support roller. The principle is to ensure that the roller blank is heated from the outside to the inside. And keep it at 200~300℃ for a sufficient time, and the roller surface must be heated evenly.

2.1.3 Control of interlayer temperature and roller body temperature uniformity during cladding welding process

Whether the interlayer temperature during the surfacing process is appropriate or not plays an important role in preventing process defects such as cracks. It must be ensured that the interlayer temperature is above 300°C (higher than the martensitic transformation temperature Ms point of the material). To avoid the martensite transformation and quenching and tempering effects of the cladding metal, the entire cladding layer undergoes martensitic transformation in the heat treatment electric furnace at the same time after welding. Only in this way can the uniformity of the structure and hardness of the cladding layer be ensured.

During the repair and surfacing process of the roll for secondary use, the temperature of the surfacing part of the base metal must be strictly implemented according to the process requirements. Always ensure the uniformity of preheating of the cladding base metal. When the temperature of the base metal overlay welding part is lower than the process requirements, the overlay welding should be stopped and intermediate heating should be performed, and the overlay welding should be continued after the temperature of the base metal overlay welding part reaches the process requirements. The entire surfacing process must be carried out on the basis of strictly maintaining the mother body at a constant temperature to ensure the strength after fusion.

As far as the back-up roller is concerned, since the alloy content of the base material and the cladding material is relatively high, the size of the back-up roll and the surfacing thickness are both large and the welding stress is high. The inter-pass temperature must be strictly controlled to maintain the coordination of multiple welding machines and the consistency of the inter-pass temperature. When surfacing the roller surface, in addition to continuing to heat and insulate the sides, flame heaters should be used to add timely heating and insulation at both ends due to rapid heat dissipation.

2.1.4 Intermediate stress relief heat treatment

Since the cladding roll is a multi-layer multi-pass welding, the rear layer has a heat treatment effect on the front layer, and the accumulation of hydrogen in the weld also increases layer by layer. Therefore, during the cladding process, when the cladding thickness reaches a certain range (unilateral cladding thickness 20-25 mm), annealing heat treatment must be performed to ensure that the cladding stress is eliminated, and then the cladding continues to the specified size.

Since the martensitic transformation points of the cladding materials of the base layer, transition layer and working layer are different, the material with the highest martensitic transformation point is usually used as the reference value of the interlayer temperature. As for the material of the back-up roller, the temperature is required to be controlled at 480~520℃, the temperature is maintained for about 20 hours, and the furnace is cooled to 400℃ before starting the furnace surfacing. Repeat the heat treatment process of cladding and improving the structure until the cladding thickness reaches the size required by the drawing.

2.1.5 Post-weld heat treatment

Dispersion strengthening treatment (tempering) is required after welding to fully precipitate carbides such as V₄C₃, Mo₂C, Cr₇C₃ for dispersion strengthening, so as to obtain a good metallographic structure of the surfacing layer after welding. Adjust the hardness of the surfacing layer and eliminate the welding stress caused by thermal expansion and contraction during the surfacing process. The main purpose of post-weld heat treatment is to improve the post-weld structure and eliminate welding stress. The back-up roller after surfacing should be cooled slowly and not exceed 250℃ before heat treatment. The initial temperature of the heat treatment furnace is about 280°C, slowly heat up to 480~520°C, and keep warm for 20 hours. In the range of 520 to 450°C, the temperature does not drop more than 10°C every 1 hour. When the temperature drops to 450°C, the power is cut off and the furnace is slowly cooled, and the furnace is discharged at room temperature.

After the back-up roller is surfacing welded, it should be cooled slowly and the temperature should not exceed 250°C before being lifted into a special heat treatment electric furnace for tempering treatment. The heat treatment furnace requires good temperature uniformity in the furnace, accurate temperature measurement, and precise temperature control process. The heating, heat preservation, and cooling of the entire heat treatment process must be strictly implemented in accordance with the process requirements.

In order to give full play to the performance of the material, high temperature tempering at 540~560℃ should be selected. Tempering temperature that is too high or too low will reduce the hardness of the surfacing layer. This is because if the temperature is too high, the decomposition of martensite and the aggregation of carbides will be intensified; if the temperature is too low, the dispersion strengthening effect will be insufficient. Reasonable tempering temperature is supplemented by sufficient tempering time. The metallographic structure of the surfacing layer after tempering is tempered martensite + carbide.

During the furnace delivery process, thick asbestos pads can be used to wrap the rolls to prevent the temperature of the rolls from dropping rapidly. At the same time, the initial temperature in the furnace should be within the required range of the support roll preheating temperature or the interlayer temperature. During the heating process, in order to ensure uniform temperature, the heating rate should be slow. During the cooling process, in order to prevent the generation of new stress, it should also be cooled slowly. After the support roller gradually cools to 50°C, it can be taken out of the oven and allowed to cool to room temperature naturally. At the same time, ensure the consistency of the temperature at the end edge and middle part of the roller body during the cooling process, and avoid the hardness difference caused by the rapid cooling of the end edge.

2.1.6 Back-up roll neck protection

The journal must be protected during tempering. The journal cannot be affected by the temperature of the heat treatment process. Once the temperature affects the journal, the surface of the journal will be oxidized, affecting the shape tolerance of this part, and even causing the concentricity to be seriously out of tolerance due to axial bending. Since the backup roller is heavy, takes a long time to weld, and is always rotating, the journals on both sides of the roller must be protected. Two pieces of forged bushings (2900/400 mm×300mm) are machined according to the journal size. After processing, they are assembled on the journals on both sides of the roller and fixed. The roller bushings are not allowed to move in series.

2.2 Surfacing process

The surfacing process parameters are shown in Table 3. According to the roller surface coloring inspection and ultrasonic inspection, the roller surface is turned (>1436 mm) to confirm that there are no cracks, pores and other defects that affect the surfacing welding.

Table 3 Surfacing process parameters

Preheating temperature ℃ Welding current AVoltage VWelding speed mm/minWire protrusion mmInterpass temperature ℃Intermediate heat treatment ℃Post-weld heat treatment ℃
350~450400~55028~32380~50020~35280~320450~500480~520

Surfacing welding is divided into base layer, transition layer and working layer. The base layer is designed with two layers, about 7 mm on one side. The transition layer is designed with two layers, about 7 mm on one side. The working layer needs to carry out multi-layer surfacing on the roller surface according to the repair requirements. After 20 to 30 mm of intermediate surfacing welding, heat treatment must be performed until the roll surface diameter reaches the agreed requirements. Before surfacing, the welding wire and flux are dried at 200°C and kept for 2 hours.

The first furnace sealing heat treatment: heat to >200℃ and keep for 4~6h, heat to >300℃ and keep for 5~10h, heat to >400℃ and keep for 5~10h, heat to >560℃ and keep for 20~30h. Cool the furnace to <400℃ and open the furnace for the first surfacing welding. Use Stellite ∅ 4 mm, 108 welding wire + 260 flux to weld the two base layers. Then use Stellite ∅04 mm, 107M welding wire + 260 flux to weld the two transition layers, and use 254 welding wire + 260 flux to weld the hard surface to ∅1491 mm × 2090 mm.

The second furnace sealing heat treatment: heat to >200℃ and keep for 4~8h, heat to >400℃ and keep for 10~14h, heat to >560℃ and keep for 8~12h. Cool the furnace to <400℃ and open the furnace for the second surfacing welding, using Stellite 254 welding wire + 260 flux to weld to ∅1545 mm×2075 mm.

The third furnace sealing heat treatment: cooling to 200℃ and holding for 5~9 hours, heating to >300℃ and holding for 8~10h, heating to >400℃ and holding for 8~10h, heating to >560℃ and holding for 5~8 hours. Cool the furnace to < 400℃ and open the furnace for the third surfacing welding. Use Stellite 254 welding wire + 260 flux to weld to ∅1594.5 mm×2065mm.

The fourth furnace sealing heat treatment: heat to >200℃ and keep for 6~8 hours, heat to >300℃ and keep for 6~8 h, heat to >400℃ and keep for 8~10 h, heat to >560℃ and keep for 8~10 h, Cool the furnace to room temperature.

The surfacing surface must be smooth and free of defects such as cracks and pores. The surfacing layer has a dense structure, no defects, and stable quality. The typical surfacing layer structure is shown in Figure 1.

(a) low magnification (b) high magnification

Figure 1 Metallographic structure of surfacing layer

3 Hardness and industrial application of cladding rollers

The hardness of the cladding roll is shown in Table 4. It can be seen that the surface hardness of the surfacing welding is 61~63HSD, which meets the requirements.

Since the repaired backup roller was first used on the machine in September 2012, it has been used 10 times in the R3 lower roller station. See Table 5 for details. The wear results show that under normal roll replacement cycles, the relative wear of the 0764# support roller is between 0.2 and 0.4 mm, which is at the same level as ordinary rollers.

Table 4 0764# Backup Roller Hardness Test Results (HSD)

Roller body12345
Bus 16262626262
Bus 26263626261
Bus 36262636362
Bus 46162626262

Table 5 0764# backup roller usage history

Serial numberDate of starting the machineDate of taking off the machineMachine diameter mmRolling tonnageDiameter before grinding mmAbsolute wear mmRelative wear mmDiameter after grinding mm
12012.09.132012.09.281583.30256001582.900.400.10541581.85
22012.10.092012.11.081581.854533181580.950.900.34081577.85
32012.12.042013.01.051577.854564131577.050.800.33121573.50
42013.01.292013.02.261573.503603621572.800.700.20141570.00
52013.03.292013.04.231570.003670171569.001.000.31401566.10
62013.08.122013.09.171566.105397411565.500.600.28651562.00
72013.10.282013.12.061562.004772811561.610.390.30171558.20
82014.01.072014.02.021558.203931571557.800.400.38051554.40
92014.03.312014.05.031554.405290141553.450.950.33071550.40
102014.06.042014.07.031550.404525981549.600.800.25781546.40

4 Conclusion

(1) The welding wire material of each surfacing layer is determined, the working layer wire composition is Cr5Ni3Mo3, and the base layer is 20CrNiMo.

(2) Turning processing must remove the residual fatigue layer on the roller surface, and the preheating temperature, interlayer temperature and post-weld heat treatment temperature should be higher than the martensitic transformation temperature of the base metal and cladding material. During the surfacing process, bushings must be used to protect the backup roll neck.

(3) The support rollers repaired by surfacing can meet the usage requirements of the R3 and R4 frames of the 2050 production line. The millimeter thickness of the cladding roller is equivalent to that of an ordinary back-up roller. The promotion and application of the surfacing repair technology for back-up rollers has good economic and social benefits.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

Share:

More Posts