Reasons for fracture of high-speed steel rolls during heat treatment

1. Fracture analysis

For centrifugally cast high-speed steel composite rolls whose outer layer material is high-speed steel and whose core material is ductile iron, the roll body sometimes breaks during the heat treatment process. There are two types of fractures, one is a fracture with a straight core and a tapered outer layer; the other is a straight fracture perpendicular to the roller surface. The fractures are mostly brown, with the darkest colour and the most severe oxidation near the bonding layer. This shows that the crack originates from the bonding layer and cracks at high temperatures. The cracks expand from the bonding layer to the inner and outer layers, and the core of the fracture is radial. Usually, the core of the roll is completely disconnected, and only a small part of the outer layer is connected, which is usually disconnected during the tempering process or when lifting out of the furnace. Some roll fractures are white and bright, and there are more serious white and bright areas near the bonding layer. In some rolls, cracks in the core of the roll will only be discovered through ultrasonic flaw detection after finishing.

 

HSS rolls fracture

 

2. Test process

 

In order to find out the reasons why the high-speed steel roll body broke during the heat treatment process, two broken rolls were sampled and analyzed. The numbers of the two rollers are A0211-403 and A0212-673 respectively.

Experimental content includes

(1) Determine the change in alloy content from the bonding layer to the centre of the roll

(2) Determine the relationship between the alloy content and the number of carbides in different parts of the roll core, as well as the morphology and changes in the metallographic structure

(3) Determine the influence of the melt-back amount of the outer layer on the core alloy content, carbide quantity and tensile strength when the roll is core-filled.

(4) Determine the expansion coefficient of high-speed steel and ductile iron

(5) Bending stress of roll bonding layer

 

3. Analysis of test results

 

1) The changes in alloy content and carbide quantity of the roll from the bonding layer to the core are shown in Table 1.

Roll numberDistance to roller surface/mmDistance to bonding layer/mmAlloy content/wt%         Number of carbides/%
CrMoVW
A0211-40342033.00
4970.7220.5900.4110.13018.69
57150.5150.4000.2430.07012.64
62200.5420.4300.2640.08013.91
74320.4710.3900.2270.09010.31
93510.5780.5100.2860.09013.14
102600.4120.3600.1670.0609.05
108660.3300.2700.1280.0507.81
115730.4370.3300.1940.06011.07
127850.4190.3300.1860.06010.55
136940.4380.3700.2040.07011.87
1451030.4140.3200.1780.06011.46
1551130.4500.3400.1980.06012.97
1651230.3550.2500.1400.05011.76
A0212-67348023.10
58100.5470.4400.3030.08010.50
65170.5160.3700.2670.0709.50
73250.5420.3900.2940.0809.53
82340.4670.3700.2460.0809.20
92440.5230.4100.2810.09010.40
105570.5470.4100.2980.08010.00
115670.5230.4200.2950.09010.16
125770.5830.4400.3260.0908.70
135870.4680.3300.2420.0708.18
1481000.5380.3900.2890.08013.60
1601120.4830.3500.2540.0808.46
1701220.5340.4100.2940.09013.56
1751270.4340.3300.2270.0809.57

 

It can be seen from the test results that from the bonding layer to the centre of the roll core, the distribution of various alloy elements is getting smaller and smaller, but there are fluctuations in concentration. This is due to the stirring of the molten iron filling the core. The number of carbides in the bonding layer of each roll is very high, some reaching more than 30%, thus drastically reducing the strength of the roll bonding layer.

2) Figure 2 and Figure 3 respectively show the distribution of carbide and alloy content in the core of A0211-403 and A0212-673 rolls.

It can be seen from the test results that the number of carbides somewhere in the core is related to the content of alloy elements in that part and corresponds to each other. This fully demonstrates that as long as the overall alloy content of the core is reduced, the number of carbides will be reduced and the strength of the roll core will be increased.

Figure 4 shows the quantity and shape of carbides in different parts of the A0211-403 roll from the bonding layer to the centre.

Similar results were obtained for the A0212-673 roll. At the bonding layer, the amount of carbide was 23.1%. At 25, 44, 87, 112, and 132mm away from the bonding layer, the amount of carbide was 9.53%, 10.40%, 8.18%, 8.46%, and 13.56% respectively.

3) As can be seen from Figure 4, the changes in the metallographic structure of different parts of the core. The amount of carbides in the bonding layer is about 30%. The content of carbides on both sides of the transition zone is higher. The thickness of the high carbide accumulation area in the bonding layer is about 2mm. On the side near the outer layer, the carbides are connected to each other in a network shape. . On the side near the core, the carbide is the primary carbide, long and thick, forming a cylindrical high carbide zone at the bonding layer. Carbide decreases from the bonding layer to the core, and the average amount of carbide in the core of the roll reaches more than 10%. The carbide is mainly strip-shaped and directional. Since the roll solidifies from the outside to the inside, the carbide grows in the radial direction. In short, the number of carbides in the core is large and no ferrite is formed.

 

4) The amount of meltback of the outer layer of the roll and the tensile strength of the core during core filling are shown in Table 2.

Roll numberMeltback amount/mmTensile strength/MPaParts
A0211-40312.5390Roll body core
490Roll neck
A0212-6738.0447Roll body core
519Roll neck

 

It can be seen from the test results in Table 1 and Table 2 that the greater the amount of melt back in the outer layer of the roll during core filling, the higher the alloy content in the core of the roll, the greater the number of carbides, and the lower the tensile strength of the core.

5) The test results of the expansion coefficient of the outer high-speed steel material and the core ductile iron material are shown in Table 3.

MaterialLinear expansion coefficient αl*106mm▪(mm℃)-1 at the following temperature
100℃200℃300℃400℃500℃600℃700℃
High-Speed Steel11.8812.7213.3413.9814.3514.5714.69
Ductile Iron10.8511.2411.5912.1812.5412.6212.70

 

It can be seen from Table 3 that the expansion coefficient of high-speed steel materials is 10% to 15% larger than that of ductile iron materials at the same temperature.

6) On the cross-section of the broken roll, cut a round bar of about Φ10mm from the core to the outer layer in the radial direction, and perform a bending test on a universal material testing machine. The load is applied to the bonding layer with a span of 100mm. The test results can be seen. Most fractures occur at the bonding layer, indicating that the strength of the bonding layer is lower than that of the outer layer and core.

 

4. Summary

 

The main reason why the roll body of a bimetallic composite high-speed steel roll breaks during the heat treatment process is that the high-temperature molten iron in the core remelts the solidified outer high-speed steel too much during core filling, increasing the total alloy content in the core. The number of carbides in the core structure, especially in the bonding layer, is too high, resulting in a sharp reduction in the strength of the bonding layer and the core of the roll.

During the heat treatment and heating process of the roll, due to the large difference in the expansion coefficients of the two materials, when the stress generated by the roll exceeds the strength of the bonding layer in the elastic range, cracks will first form near the bonding layer, and during the subsequent heating process, the cracks will move towards the core. During the quenching and cooling process, the cracks expand to the outer layer, and eventually the entire roll breaks during tempering.

It can be seen that controlling the melt-back amount of the outer layer of high-speed steel during core filling is the key to solving the problem of high-speed steel roll fracture. It is necessary to ensure the bonding quality of the bonding layer of the high-speed steel composite roll and to limit the content of the bonding layer and core alloy. More importantly, The most important thing is to ensure that ferrite must be formed in the core structure of the roll. Therefore, when producing larger-diameter high-speed steel composite rolls, three-layer composites must be used to avoid the remelting of a large amount of high-temperature hot metal in the core to the outer high-speed steel and reduce the roll bonding layer. and the number of carbides in the core, thereby avoiding roll body breakage during the heat treatment of high-speed steel rolls.

 

Source of the article: Zhang Zili, Gao Chunli, open collar – “Analysis of Causes of Fracture of High-Speed Steel Roller Heat Treatment”

 

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