Resurfacing and Hardfacing Continuous Casting Rolls

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This article is about the Continuous Casting Rolls resurfacing process using the Arc Welding researched by BCC D-Techs(The R&D department of BCC innovation center) - Surfacing solutions to improve the Life of Continuous Casting Rolls by Restoration, Welding, Hardfacing technology . It indicates the base material, welding parameters, requirements during welding,  heat treatment after welding and provides some most outstanding features of Alloy Wire.

2.0    PROCESS AND BASE MATERIALS

2.1 Process

Fluctuating roll temperature, water spray, low pH levels and abrasion all lead to harsh conditions for continous casting roll.. The combination of these stresses causes rapid wear on the surface of rolls and that's the reason Continuous Casting Rolls need wear solutions for lifespan. And it also is base information for the decision of Welding materials, technology and method.

2.2 Roller's Base Material

This one considers the restoration, hardfacing process of the casting roll in steel industry. The type of material used in this process is mentioned in EN 10269, 16CrMo44, 25CrMO4, 42CrMo4 or equivalent base. The composition of the steels is mentioned in Sheet 1

Sheet 1: Material's chemical composition

3.0 WELDING OVERLAY MATERIAL

Welding wire D430, D414 are recommended for resurfacing of these rollers.

In which, D430 welding wire is used for primer welding. Welding metal is specially used for rollers with a 42CrMo4(base material) or equivalent composition which has a carbon content of at least 0.3%. D414N is used for the next overlays. Depending on the desired, the number of layers can be 2 or more.

If it's 16CrMo44 and 25CrMo4 rollers, D430 can be used directly after the surface has been thoroughly cleaned before welding without the use of a primer.

D430 contains low carbon, low silicon, low chromium alloy, low molydenum, low another weld metal. and medium carbon D414 wire, martensitic steel weld metal with added Nickel, Molydenum and Vanadium. The weld metal also includes the certain amount of Nitrogen. This combination gives the weld overlay a medium-carbon heat treatment structure similar to that of martensitic steel. Therefore, it is designed for hard and tough materials. The addition of Nitrogen while welding will bring harder and more wear-resistant in applications working under high pressure and high temperature, Temperature cycle.

4.0 TECHNIQUES AND PARAMETERS

The advantage of SAW alloy welding wire is that the higher ratio welding overlay (width, height). The overlay also have no difference between the welding top and welding bottom.

An outline plot of the fillet surface is shown in Figure 1. In general, the height of the overlay (SAW) process will be lower than that of others welding process.

Figure 1: Comparison of crown height between SAW and GMAW

For the Weld Overlay, the overlapping of adjacent lines can significantly reduce the deformation of the welded structure (Figure 1). Therefore, locating the wire during welding will ensure a higher weld overlap (approximately 50%) to reduce the deformation of the structure shown in figure 2.

Figure 2 -Effect of weld overlap

5.0 HEAT TREAMENT AND METHODS OF REDUCE RESIDUAL STRESS

Welding is a fabrication process through the application of heat, which will cause residual stress(mainly the result of thermal expansion) after the welding work is completed. The purprose of "Heat Treatment" is to reduce residual stress and rearrange the structure of weld metal. With small rollers(Mill rolling) are possible to use heat treatment furnace to do this. For large size (Support Roller) must be heat treated and temperature controlled by a heat treatment machine(6 channel program controllers) after welding. The temperature control process is carried out as the following:

• Heating to 400-450 in steady for 6-8 hours.
• The initial temperature of Roller must be lower than 100 degrees.
• Heating and Cooling speed is maximum 100 degrees/hour

The hardness after welding, heat treatment is shown in figure 4. The hardness after heat treatment is 40-42 HRC. Temperature control is very important. As if the temperature exceeds the upper limit (>450 degrees), the microstructure of weld will be roughened and soften the weld metal.

Figure 4: Effect of heat treatment and surface hardness of welding overlay

6.0 STEP BY STEP OF WELDING PROCESS

Recommendation process for Build-up, Hardfacing of Support Roller:

6.1 Surface preparation.:

Rollers surface should be cleaned to free of any loose material, dirt, oil, grease, etc.…

Do inspect both on the surface and inside the structure by Ultrasonic Testing. In case there are cracks, it should be repaired with welding filler wire.

6.2  Stress relieving before welding

Roller has been existing residual stress and it surface has been partially hardened. And that's the reason Roller should be destressed by 400-450 degrees for approximately One hour per inch of thickness (1h/1inch) and being cooled slowly to reduce the pre-existing residual stress.

6.3 Heating and maintain the tempurature:

Heat to a minimum of 250 degrees before welding and keep in steady temperature (not to exceed 250 degrees) throughout the welding process.

6.4 Welding

Depending on the roller composition, welding procedure used with D430 as primer and D414 as filler weld.

With optimal welding parameters, each layer of welding will be 2-3mm thicker on diameter. Welding parameters as mentioned in Welding Procedure.

The temperature should be maintained at 250 degrees in the whole welding process.

6.5  Stress relieving after welding:

The roller must be heat treated to reduce residual stress and rearrange the microstructure of weld metal..

6.6 Machining

After heat treatment, it will be machined to desired dimension.  Surface parameters: Dimensions, surface roughness, concentricity must be strictly controlled to ensure the operating after, it directly affects the service life.

6.7 Quality control:

Some tests will be taken:

• Non-Destructive Testing PT & UT, before surface machining.
• Size testing.
• Roundness and Concentricity testing.

 7.0  PROCESS NOTES

To achieve the expected results: Please! Follow the instruction and warning during process.

1. Carefully prepare the surface.
2. Reduce roller residual stress before welding
3. Heating in steady to 250 degrees throughout the welding process.
4. Rollers on the inside of the furnace must be placed in a reasonable position: deep inside or in the middle in oder to get the steady heat.
5. Strict control of the heat treatment process ensures the uniformity of the entire roller surface. The heating and cooling rate does not exceed 100 degrees/hour.
6. During the heat treatment, after cooling down to 100 degrees, the roller will be allowed to air-cool to room temperature. Hardness is exactly achieved only after cooling to room temperature (25-30 degrees Celsius).

8.0 OUTSTANDING FEATURE OF D-430 AND D414 WELDING WIRE

1. Low carbon (<0.05%), Low silicon (<0.4%), with the addition of Chromium, Nickel and Molybdenum.
2. The filler surface is mostly same, weld joint is better and the hardness is uniform
3. Filler weld metal will form a stable microstructure after stress reduction treatment without cracks
4. Excellent wear resistance
5. Filler surface have abrasion resistant capacity in normal climate condition.
6. Well welding rate with normal operating parameters (approximately 6.0 kg/1 hour arc with 2.4mm diameter wire)
   

Nguồn: BCC D-Techs

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