16 examples of tool material defect analysis
Materials are the basis for manufacturing long-life tools. In actual production, various forms of material defects are often encountered. Now I will show them to my readers. I hope everyone will pay more attention to raw materials. The following are 16 examples.
1. Looseness of raw materials
After acid etching test of steel, it was found that some areas on the surface of the sample were not dense, and there were some visible voids. These voids showed dark spots with irregular colors, darker and lighter than other parts, called looseness. If the looseness is concentrated in the central part of the sample, it is called central looseness; if the looseness is more evenly distributed on the surface of the sample, it is called general looseness. Both GB/T9943-2008 "High-speed Tool Steel" and GB/T1299-2014 "Tool Steel" have clear regulations on the looseness of steel, some suppliers still sell substandard steel.
Porosity has a great influence on the strength of steel, and the main hazards are as follows:
(1) Porosity significantly reduces the strength of the steel, and it is easy to crack during thermal processing such as forging, and it is also easy to form cracks in the porosity during heat treatment.
(2) Due to the looseness of the material, the made tools are easily worn and the surface is not smooth.
Because porosity has a certain effect on the performance of steel, tool steel has strict requirements on the allowable porosity level. Figure 1 and Figure 2 are φ90mm W18Cr4V (hereinafter referred to as W18) steel raw materials and forging loose and loose cracking morphology (1:1 HCl aqueous solution hot etching). Figure 3 is a picture of W18Cr4V steel groove milling cutter due to severe heat treatment cracking due to looseness (1:1 HCl aqueous solution thermal etching).
2. Residue of Shrinkage cavity
When the steel ingot is poured, the molten steel shrinks in the central part during the condensation process, a tubular hole is formed, which is called shrinkage cavity. Shrinkage holes are generally located near the riser of the head of the steel ingot, and should be removed when the ingot is finished. The remaining part is called the shrinkage hole residue if it is not completely removed. It stands to reason that the shrinkage cavity should be completely removed, but steel mills often pursue the yield rate and leave residues, leaving irreparable disasters to the follow-up process. Figure 4 shows the residual and severe porosity of the φ70mm W18 steel shrinkage cavity (1:1 HCl aqueous solution hot etching), and Figure 5 shows the cracks formed after the φ70mm W18 steel shrinkage cavity residual after rolling (1:1 HCl aqueous solution hot etching). A few years ago, in a company, people found residual shrinkage holes when they were cutting φ75mm M2 steel.
3. Surface cracks
Longitudinal cracks found on the surface of high-speed steel raw materials are very common. The reasons may be as follows:
(1) When the steel is hot rolled, the surface cracks are not completely removed, or the surface is scratched by the die hole, stress concentration occurs during the cooling process, and cracks are caused along the scratch line.
(2) During hot rolling, folds are caused by poor die holes or large feed rates, which will cause cracks along the fold lines in the following processing.
(3) During hot rolling, the rolling stop temperature is too low, or the cooling rate is too fast to cause cracks.
(4) Surface cracks are often found in W18 steel 13mm×4.5mm flat steel rolled by a company in the cold winter, indicating that the climatic effects has a impact on the formation of cracks , but there is no crack when rolled at other times of the same steel grade and same specification. Figure 6 shows the surface crack of φ30mm W18 steel (1:1 HCl aqueous solution hot etch), with a depth of 6mm.
4. Cracks in the center of the raw material
In the hot rolling process of high-speed steel, due to the excessive deformation, the core temperature does not drop but rises. Under the action of thermal stress, the center of the material cracks. Figure 7 is a picture of φ35mm W18 steel center cracking (1:1 HCl aqueous solution hot etching). When a tool factory saw high-speed steel raw materials, center cracks were common. This crack is extremely annoying, invisible and intangible.
The uneven chemical composition of the alloy formed during the solidification process is called segregation, especially the uneven distribution of impurities in carbon and steel, which will have a great impact on the performance of the steel. Segregation can be divided into: ①Dendrite segregation. ② Density segregation: The density of the constituent phases in the alloy is very different. During the solidification process, the heavy one sinks and the light one floats. ③Regional segregation: caused by local accumulation of impurities in ingots or castings.
Figure 8 is a metallographic sample of W18 steel after quenching (etched in 4% HNO3 alcohol solution). It is found that there is a cross-shaped pattern. After chemical composition analysis, the carbon content of the matrix part is lower, and the carbon content of the cross-shaped part is higher.Therefore, it is believed that the cross shape is a kind of uneven chemical composition, which is caused by the segregation of carbon and alloy components, which forms a cross shape after rolling.
If there is serious regional segregation, the strength of the steel will be reduced, and it will be easy to crack at the segregation place during hot working.
6. Carbide unevenness
The degree to which eutectic carbides in high-speed steel are broken during hot press processing is called carbide unevenness. The greater the amount of deformation, the higher the degree of carbide fragmentation, and the lower the level of carbide unevenness. When the carbides in the steel are serious, such as thick bands, nets, and large carbides, they will have a greater impact on the quality of the steel. Therefore, strict control of it is necessary to ensure the quality of high-speed steel tools condition. Figure 9 shows the influence of carbide unevenness on the bending strength of W18 steel. It can be seen from the figure that the flexural strength of grades 7-8 with unevenness is only 40%-50% of grades 1-2 steel, when the pressure is reduced to 1200-1500MPa, the strength of high-speed steel is only equivalent to the level of the higher toughness grades in cemented carbide; transverse performance is about 85% of the longitudinal performance. The concentration and band-like distribution of carbides will also cause uneven quenched grains and uneven dissolution of carbides. The former will increase the tendency of overheating, and the latter will reduce the secondary hardening ability.
Severe carbide inhomogeneity can easily cause cracks and overheating during hot working, and make the finished tools chipped during use. Figure 10 shows the quenching cracking of W18 steel coarse band carbides (etched by 4% HNO3 alcohol solution).
When steel is hot rolled or annealed,if the heating temperature is too high and the holding time is too long, it will cause crystal grains to grow, and during the slow cooling process, carbides precipitate along the grain boundaries to form reticulated carbides. The reticulated carbide greatly increases the brittleness of the tool and is prone to edge chipping. In general, the existence of complete reticulated carbide is not allowed in steel. The inspection of reticulated carbides should be carried out after quenching and tempering. Figure 11 shows the reticulated carbide of T12A steel (etched in 4% HNO3 alcohol solution), and Figure 12 shows the morphology of the reticulated carbide of 9SiCr steel (etched in 4% HNO3 alcohol solution). It can be seen that there is severe overheating during annealing.
In some tool factories, when turning or milling high-speed steel, the cutting tools will encounter hard materials and be damaged. In general, due to the high cutting speed and high noise during turning, this defect is not easy to be found, but during milling, it is possible to observe lumps and strange chaos: for example, when twist drilling and milling, it is found that the milling cutter has been in service to a certain level. The position cannot be processed continuously, squealing sound is produced, and the tool is severely burned. People cut this material and found that there are bright blocks visible to the naked eye. After the hardness test, the hardness of this bright block is extremely high, reaching 1225HV, and the non-hardened area is in a normal annealing state. We call it a "lump". Due to the existence of lumps, the tool is damaged and cutting is difficult. The formation of hard lumps is estimated to be caused by the segregation of chemical components during the smelting process. The hard lumps themselves may be a kind of high-hardness composite carbide, or they may be stored in steel due to the addition of refractory alloy blocks during the smelting process. Figure 13 is a photo of the macrostructure of a lump of W18 steel (etched by 4% HNO3 alcohol solution). The white block is the lump and the gray-black block is the bit groove.
Inclusions are a common defect in steel. According to their properties, they can be divided into metallic inclusions and non-metallic inclusions. Metal inclusions are formed because the ferroalloy is not fully melted during the smelting process, or because foreign inclusions that flow in during the pouring process remain in the steel ingot. There may be two types of non-metallic inclusions: ① Foreign inclusions--mainly because the pouring system is not clean; the refractory mud on the equipment is peeled off; the charge used is not pure and so on. ② Products produced and precipitated due to chemical reactions in the smelting process. Figure 14 is a photo of metal inclusions found in W18 steel, and Figure 15 is a photo of non-metallic inclusions causing cracking during quenching (etched by 4% HNO3 alcohol solution).
It has been ascertained that inclusions are very harmful to the quality of steel. They divide the steel matrix and reduce the plasticity and strength of the steel, making the steel easy to form cracks at the inclusions during rolling, forging, and heat treatment. Inclusions can also cause steel fatigue and cutting and grinding difficulties, so tool steel should have certain requirements for inclusions.
In the process of steel smelting, due to component segregation, the carbides are unevenly distributed, or the carbides contained in the iron alloy are not completely melted, resulting in large angular carbides, which are preserved without being crushed after forging and rolling. The presence of large carbides will increase the brittleness of the tool and easily cause chipping. In the heat treatment process, due to the enrichment of large carbides and alloying elements, defects such as overheating, insufficient tempering and even cracking along the grain boundary are easily generated. Figure 16 is a picture of overheating of quenching caused by segregation of surrounding components of large carbides (etching in 4% HNO3 alcohol solution).
11. liquation carbonide
During the solidification process of liquid metal, due to the segregation of carbon and alloying elements, the segregation will precipitate large carbides in the liquid during cooling, which is not easy to be eliminated in the subsequent normal processing. It is in the form of large carbide bands along the The rolling direction of the steel exists in the steel. This kind of segregation is called liquid separation. Figure 17 is the CrMn liquid analysis picture (4% HNO3 alcohol solution etching).
Because the annealing temperature is too high and the holding time is too long, the carbides are easily decomposed into free carbon or graphite during the long slow cooling process of the steel. Figure 18 shows the graphitic carbon structure of T12A steel (etched in 4% picric acid alcohol solution).
The precipitation of graphite carbon greatly reduces the strength and wearability of steel. This material is not suitable for manufacturing tools and important parts. Steel with severe graphite carbon has a black fracture. The content of graphite can be determined qualitatively and quantitatively by chemical analysis, and its shape and distribution can be observed by metallographic method. There will be more ferrite structures around graphite.
13. Unqualified mixture and ingredients
Mixing materials in tool and mold manufacturing enterprises is the norm, it is a fault of management, and it is a low-level defect. Mixing includes three aspects: mixing steel number, mixing specification, mixing furnace number, especially mixing furnace number is very common, causing many "unjust, false and wrong cases" to heat treatment, and there is no place to appeal. Unqualified tool material components occur from time to time. Some high-speed steel components do not meet the GB/T9943-2008 "High-speed tool steel" standard, especially carbon, which is either high or low. W6Mo5Cr4V2Co5 belongs to the HSS-E category. Because the C content is lower than the lower limit of the standard, the hardness cannot reach 67HRC after heat treatment. It even can not be called high-performance high-speed steel.Since it belongs to the HSS-E category, the steel mill must ensure that the steel can reach 67HRC or higher. As for the use of such high hardness for the tool, it is an internal matter of the tool factory and has nothing to do with the steel mill. However, it is the steel mill's fault if their steel does not reach 67HRC. There are also many cases of unqualified die steel composition, and disputes continue.
14. Decarbonization of raw materials
The country has standards for steel decarburization layers, but steel suppliers often supply materials that exceed decarburization standards, causing tool factories to suffer great economic losses. For materials with a decarburized layer, the surface hardness of the tool decreases after quenching, and the wear resistance is poor. Therefore, the decarburized layer of steel must be completely removed during machining, otherwise it will bring a series of quality hazards. Figure 19 shows the decarburization morphology of W18 steel raw material (etched in 4% HNO3 alcohol solution). The decarburization zone is needle-shaped tempered martensite, and the non-decarburized zone is quenched martensite + carbide + retained austenite. Figure 20 shows the decarburization of M2 steel. Figure 21 shows the decarburization of T12 steel (4% HNO3 alcohol solution etching), the fully decarburized layer is ferrite, the transition zone is carbon-lean tempered martensite, and the non-decarburized zone is tempered martensite + carbide .
We select a company’s W18 steel 13mm×4.5mm flat steel, quenched in salt bath at 1210℃, 1230℃, 1270℃, the heating time is 200s, and the grain size is 10.5, as shown in Figure 22. The hardness after quenching is 65～65.5HRC, and the hardness does not increase but decreases after tempering at 550℃ for three times. This result is very strange, so I call it "a mystery."
The root cause of the anecdote is that carbide is teasing us, meaning that when carbide is heated, it does not dissolve into austenite, nor does it precipitate during the tempering process. Briefly, if the carbide can’t get in or out, how could it be possible to be hardened twice?
Surface defects are visible to the naked eye, the dimension of steel must be mentioned in the contract. But the actual supply may has different lengths and sizes, ultra-thin steel surface pits, corrosion pits, roundness, Horseshoe-shaped substance, excessive steel plate unevenness, uneven thickness and many other surface defects.
There are many examples of steel defects. I hope everyone will pay attention to choosing materials. Materials are the foundation. If the foundation is not strong, the ground will be shaken. Can poor materials be made into good tools? Of course not!
Reprinted from magazine“Metal Processing-Thermal Processing” Issue 9 of 2019 page 51-55.