High-speed steel composite rolls are favoured by users due to their high hardness and good wear resistance, and their application in the steel rolling industry has developed rapidly. High-speed steel roll inspection is an important link to ensure product quality and later use effects. This article discusses the product inspection items and practices of high-speed steel rolls based on years of experience in producing high-speed steel composite rolls.
1. Production of high-speed steel rolls
This article mainly discusses the production of small and medium-sized centrifugal composite high-speed steel rolls. The diameter is generally less than φ600mm. The working layer is made of high-speed steel and the core is mainly made of ductile iron. It is produced by horizontal centrifuge casting. In order to increase the strength of the bonding layer and core and reduce the diffusion of strong carbide elements in the working layer to the core, a transition layer is added between the working layer and the core when necessary.
The main production and inspection procedures of high-speed steel rolls are as below
Raw material inspection – smelting of molten steel in the working layer of high-speed steel rolls and molten iron in the core – detection of chemical composition of working layer and core materials – centrifugal compounding of rolls – size inspection of roll blanks – Roll annealing heat treatment – roll rough machining and dimensional inspection – ultrasonic flaw detection – roll normalizing and tempering heat treatment – roll hardness, metallographic structure and ultrasonic flaw detection – roll finishing – finished product dimensional inspection – packaging Inspection – Warehousing.
2. Inspection of high-speed steel rolls
According to the characteristics of the production process of high-speed steel rolls, the quality inspection of high-speed steel rolls mainly includes the following 6 items.
(1) Raw material inspection
(2) Inspection of the chemical composition of rolls
(3) Roll hardness inspection
(4) Roll metallographic structure inspection
(5) Roll ultrasonic flaw detection inspection
(6) Roll geometric size inspection and roll size inspection are the same as other types of rolls, and square drawing inspection is used.
1) Roll raw material inspection
During the smelting process of hot metal in the working layer of high-speed steel rolls, a large amount of cast pig iron, scrap steel, and ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten, ferroniobium and other ferroalloys are used. For cast pig iron, ferrochrome, ferromolybdenum, ferrovanadium, ferrotungsten and ferroniobium, the corresponding national standards can be implemented. Since there are no suitable scrap acceptance criteria. The composition of scrap steel purchased from the market varies widely and is relatively complex. The main purpose of inspection should be to control the content of phosphorus, sulfur and other impurities in the raw materials to improve the quality of the molten steel in the working layer of high-speed steel rolls.
2) Inspection of the chemical composition of rolls
The chemical composition of high-speed steel rolls is a necessary condition to ensure good overall performance of the rolls. A Swiss ARL-3460 spectrometer is used to detect the chemical composition of alloy elements. In order to ensure the accuracy of detection, a special spectral detection curve for high-speed steel is established. Carbon and sulfur elements are detected using the German-made ELTRACS800 carbon and sulfur analyzer, which has ideal detection efficiency and accuracy.
3) Roll hardness inspection
High-speed steel roll hardness testing includes roll working layer hardness testing and roll neck hardness testing. The working layer testing method is to detect at least 2 busbars in the circumferential direction of the roll surface, and each busbar is tested at 3 points, namely at both ends and the middle of the roll body. The roller surface hardness unevenness is required to be less than HSD4, and the roller body surface hardness is required to reach HSD78~90. The hardness of the roll neck is only tested at one point on each of the two bus lines of the upper and lower roll necks. For ductile iron, the general hardness is HSD35~48.
4) Roll metallographic structure inspection
The metallographic structure inspection of the roll mainly includes the metallographic structure inspection of the working layer and the metallographic structure of the roll neck. The normal working layer structure of high-speed steel is carbide + tempered martensite + bainite. The metallographic structure of the roll neck is spherical graphite + bull’s-eye ferrite + fine pearlite structure + a small amount of phosphorus crystal structure.
The low hardness of the working layer has occurred during the roll manufacturing process. In the metallographic structure of high-speed steel rolls with low working layer hardness, the matrix structure is mainly sorbate and bainite, and the dispersion strengthening of granular carbides is not obvious after tempering.
5) Roll ultrasonic flaw detection inspection
Roller flaw detection mainly includes magnetic particle testing, penetrant testing, eddy current testing and ultrasonic testing. High-speed steel rolls generally use ultrasonic flaw detection to mainly detect whether there are cracks, slag inclusions, poor bonding and other defects in the working layer and bonding layer. During the ultrasonic flaw detection process, when it is difficult to judge the waveform of the bonding layer, destructive experiments are required, and the test results of the metallographic structure and tensile strength are combined to determine the bonding quality of the roll.
When inspecting high-speed steel rolls with good quality, the display screen shows a primary wave and no other clutter.
When inspecting high-speed steel rolls with fracture defects, cracks in high-speed steel and ductile iron centrifugal composite rolls generally appear as transverse fractures in the core of the roll. The cracks propagate from the core to the working layer, sometimes there is one crack, sometimes there are 2 to 3 cracks, and there is almost no axial cracking. When the roller surface is inspected with an oblique probe, a defect wave appears on the display screen. When the roller body is inspected axially with a straight probe, a defect wave appears with a wave height greater than 100%. The size of the crack determines whether a bottom wave appears.
When inspecting high-speed steel rolls with cracks in the bonding layer, the cracks in the bonding layer of the roll mainly expand in the axial direction of the bonding layer. When detecting with a radial straight probe on the roll surface, primary or secondary waves exceed 100%, and there are even 3 to 5 reflections. The wave and wave height decrease successively, but the waveform detected by the roller surface axial straight probe is not obvious.
When inspecting high-speed steel rolls with slag inclusion defects, there are two types of slag inclusion defects in high-speed steel rolls: point-like slag inclusion and area-type slag inclusion, which mainly appear in the bonding layer or a point or a local area below the bonding layer. Slag inclusions are mainly glass slag, and occasionally paint or sand. When testing with a radial straight probe on the roller surface, the waveform is generally greater than 100% of the primary wave. The height of the secondary echo is uncertain, and the depth and thickness of the slag inclusion must be judged based on the height and position of the primary echo. Depending on the diameter of the roll, scattered point-like slag inclusions are allowed, but large-area slag inclusions are not allowed.
When inspecting high-speed steel rolls with poor bonding, the primary wave mainly shows a high level, greater than 100%, or slightly lower than 100%. Some secondary waves appear, and some do not. During the entire roll bonding layer detection process, the area where the above-mentioned waveform appears is larger, and other areas show well-integrated waveforms.
During the roll flaw detection process, the entire roll body bonding layer showed local primary wave heights greater than 100%, some locations with secondary wave heights of 40%, and other areas with primary wave heights of 40% to 60%. The determination of the roll bonding quality is controversial. In order to ensure the quality of the roll, we cut slices from the roll body for destructive testing, cut tensile strength samples at different wave heights to check the strength of the bonding layer, and also conducted the metallographic inspection. The metallographic examination results show that flake graphite appears near the core ductile iron bonding layer, similar to the black line of the infinite IV roller. It is this flake graphite that causes the interface wave to appear during flaw detection. The test results of the tensile strength of the bonding layer are shown in the table. It can be seen that it can meet the use requirements of the roll. After strengthening the treatment of the molten iron, this waveform has never appeared again.
Test results of tensile strength of defective high-speed steel roll bonding layer
|Flaw detection position
|One wave height is 100%
|One wave height is 100%
|Secondary wave height is 40%
|One wave height is 40%~60%
Due to the different manufacturing methods of high-speed steel rolls, the detection methods are also different. In particular, the metallographic structure of the working layer and the ultrasonic flaw detection of the bonding layer are the most important aspects to ensure the use effect of high-speed steel rolls and should attract the attention of manufacturers and users.