Materials are the basis for manufacturing long-life tools.
In actual production, various forms of material defects are often encountered.
Today we will share the16 types of steel defects with you to make you pay attention when choosing raw materials.
01. Raw materials porosity
After the acid etching test of steel, it is found that some areas of the surface of the sample are not dense, and there are some visible voids.
These voids show dark spots with irregular color shades compared to other parts, which are called porosity.
If the porosity is concentrated in the central part of the sample, it is called central porosity;
If the porosity is more evenly distributed on the surface of the sample, it is called general porosity.
Both in GB/T9943-2008 High-Speed Tool Steel and GB/T1299-2014 Tool Steel have clear regulations on the porosity of steel, but they often exceed the standard supply.
Porosity has a great impact on the strength of steel, the main hazards are as follows:
(1) Porosity significantly reduces the strength of the steel, it is easy to crack during hot working such as forging and is also easy to form cracks in the porosity during heat treatment.
(2) Due to the porosity 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 (abbreviated as W18) steel raw materials, forging porosity and porosity cracking patterns (heat etching with 1:1 HCl).
Figure 3 shows a picture of a W18Cr4V steel slotted milling cutter cracked by severe heat treatment due to sparing (heat etching with 1:1HCl).
Figure 1 Central porosity
Figure 2 Cracks of central porosity steel during forging billets
Figure 3 Cracks of slotting cutter material due to porosity during heat treatment
02. Shrinkage residue
When the ingot is cast, the liquid steel shrinks in the central part of the condensation process to form a tubular hole, which is known as the shrinkage.
Shrinkage is generally located near the feeder in the head of the ingot, and it should be removed when the billet is formed.
The remaining portion that cannot be completely removed is called shrinkage residue.
It is reasonable to remove the shrinkage completely, but steel mills often pursue the success rate and leave a residue, leaving an irreversible disaster for the subsequent process.
Figure 4 is φ70mm W18 steel shrinkage residue and serious porosity picture (heat etching with 1:1 HCl), Figure 5 is φ70mm W18 steel shrinkage residue after rolling to form cracks (heat etching with 1:1 HCl).
A few years ago, when a company sawed φ75mm M2 steel, it was found that there were also shrinkage residues.
Figure 5: Cracks caused by W18 steel shrinkage
03. Surface crack
Longitudinal cracks found on the surface of high-speed steel raw materials are very common.
The reasons may be as follows:
(1) In hot rolling, stress concentration occurs during the cooling process, and cracks are caused along the scratch line because the surface cracks are not completely removed, or the surface is scratched by the die hole.
(2) During hot rolling, folds due to poor die holes or large feed rates will cause cracks along the fold lines in the subsequent processing.
(3) During hot rolling, cracks could be produced because the rolling stop temperature is too low, or the cooling rate is too fast.
(4) Surface cracks are often found on W18 steel 13mm×4.5mm flat steel rolled in the cold winter, indicating that the cracks also have climatic effects, 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 (heat etching with 1:1 HCl), and the depth reaches 6mm.
Figure 6 Surface crack
04. Cracks in the center of raw material
During the hot rolling process of high-speed steel, due to excessive deformation, the central temperature rises instead of decreases.
Under the effect of thermal stress, the crack occurs in the material center.
Figure 7 is the center crack of φ35mm W18 steel (hot etching with 1:1HCl).
It is common to see the central crack when sawing high-speed steel raw materials in the tool plants.
This crack is harmful because it is invisible and untouchable, the only way to see the true face is flaw detection.
Figure 7 Central crack
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:
② Density segregation
The density of the constituent phases in the alloy is very different.
During the solidification process, the heavier sinks and the lighter floats.
③ Regional segregation
It is caused by local accumulation of impurities in ingots or castings.
Figure 8 shows the quenched metallographic sample of W18 steel (etched by 4% HNO3 alcohol solution), which is found that there is a cross-shaped pattern.
After the chemical composition analysis, the carbon content of the matrix part is lower, and the carbon content of the cross-shaped part is higher.
Therefore, the cross shape is considered to be a kind of uneven chemical composition.
It is the square segregation 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 is easy to crack at the segregation place during hot working.
Figure 8 Cross-shaped segregation (3×)
06. Carbide nonuniformity
The extent to which the eutectic carbides in HSS are broken down during the hot press process is called the carbide nonuniformity.
The greater the deformation, the higher the degree of carbide fracture, and the lower the level of carbide nonuniformity.
When the carbides in the steel are severe, such as coarse ribbon, mesh, and large carbide buildup, it has a major impact on the quality of the steel.
Therefore, strict control of it is necessary to ensure the quality of HSS tools.
Figure 9 shows the effect of carbide nonuniformity on the bending strength of W18 steel.
It can be seen from the figure that the bending strength in grades 7-8 with nonuniformity is only 40%-50% of grades 1-2, which is reduced to 1200-1500MPa and it is only equivalent to the level of the higher toughness grades in cemented carbide; the horizontal performance is about 85% of the vertical performance.
The concentration and band-like distribution of carbides will also cause uneven quenched grains and uneven dissolution of carbides, which will increase the tendency of overheating and reduce the secondary hardening ability, respectively.
Figure 9 The influence of carbide nonuniformity on the bending strength of HSS (W18Cr4V)
Severe carbide nonuniformity can easily cause cracking and overheating during hot working, which make the finished tool tipped in use.
Figure 10 shows the quenching crack of W18 steel coarse zonal carbides (etching with 4% HNO3 alcohol solution).
Figure 10 Coarse zonal carbide
07. Network carbide
Steel in hot rolling or annealing, due to the high heating temperature, long holding time that results in grain growth, and in the slow cooling process of carbide precipitation along the grain boundaries, a network carbide is formed.
The network carbide greatly increases the brittleness of the tool and is prone to chipping.
In general, the existence of complete network carbide is not allowed in steel.
The inspection of network carbides should be carried out after quenching and tempering.
Figure 11 shows the network carbides of T12A steel (etched in 4% HNO3 alcohol solution), and Figure 12 shows the morphology of the network carbides of 9SiCr steel (etched in 4% HNO3 alcohol solution), which can be seen that there is severely overheating during annealing.
Figure 11 T12A Steel Mesh Carbide (500×)
Figure 12 9SiCr Steel Mesh Carbide (500×)
08. Carbide caked mass
There are some tool mills that perform HSS turning or milling, the tool encounters a hard substance and is 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 milling slots with twist drills, it is found that the milling cutter cannot continue machining in a certain position, resulting in a squeaking sound and severe burnout of the tool.
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-hard areas are in a normal annealing state.
We call it a “caked mass”.
Due to the existence of caked mass, 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 caked mass 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 the macrostructure of a caked mass in W18 steel (etched by 4% HNO3 alcohol solution).
The white one is the caked mass, and the gray and black are the bit grooves.
Figure 13 The macrostructure of W18 steel caked mass (20×)
Inclusions are a common defect in steel, which can be divided into metallic inclusions and non-metallic inclusions according to their properties.
Metal inclusions are formed because the ferroalloy is not fully melted during the smelting process, or because foreign metal foreign bodies that flow in during the casting process remain in the steel ingot.
There may be two types of non-metallic inclusions:
① Endogenous inclusion: mainly because the pouring system is not clean; the refractory mud on the equipment is peeled off; the charge used is not pure, etc.
② Products produced and precipitated due to chemical reactions in the smelting process.
Figure 14 is metal inclusions found in W18 steel, and Figure 15 is non-metallic inclusions causing cracks during quenching (etched by 4% HNO3 alcohol solution).
Figure 14 Metal inclusions
Figure 15 Cracking caused by non-metallic inclusions during quenching (400 x)
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 as well as cutting and grinding difficulties, so tool steel should have certain requirements for inclusions.
10. Bulk carbide
In the process of steel smelting, due to component segregation, the carbides are unevenly distributed, or the carbides in the iron alloy are not completely melted, resulting in large angular carbides, which are preserved without being crushed after forging.
The presence of bulk carbides will increase the brittleness of the tool and easily cause tipping.
During 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 likely to occur.
Figure 16 is the overheating of quenching caused by segregation of surrounding components of large carbides (etching in 4% HNO3 alcohol solution).
Figure 16 Overheating caused by segregation of components around bulk carbides during quenching (500×)
11. Carbide liquation
Liquid metal in the solidification process, due to the segregation of carbon and alloying elements, the cooling causes the segregation to precipitate large blocks of carbide in the liquid.
It is not easily eliminated during subsequent normal processing and is present in the steel in the form of bulk zoster carbide in the direction of the rolling of the steel.
This segregation is called liquation.
Figure 17 shows CrMn liquation (etched with 4% HNO3 alcohol solution).
Figure 17 Carbide liquation (500×)
Steels with liquation are very brittle, the continuous matrix of metal is cut and strength is reduced.
Previously, liquation was common in CrWMn and CrMn steels, and if they were used to make gauges, it was difficult to obtain a smooth surface.
12. Graphite carbon
As the annealing temperature is too high, holding time is long, so that the steel in the long slow cooling process, carbide easily decomposed into free carbon, i.e. graphite.
Figure 18 is T12A steel graphite carbon microstructure (etched in 4% alcohol bitter acid solution ).
Figure 18 Graphitic carbon microstructure of T12A steel (500×)
The precipitation of graphite carbon greatly reduces the strength and wear resistance of steel, but this material is not suitable for the manufacture of knives and important parts.
Graphite carbon-heavy steel is with black fractures.
Graphite content can be chemical analysis for qualitative and quantitative determination, its shape and distribution of metallographic methods can be observed.
More ferrite tissue will appear around the graphite.
13. Failure of mix and composition
Mixing of materials in tool and mold manufacturing enterprises is normal, which is a fault of management and a low-level defect.
Mixed material includes three aspects: mixed steel, mixed specifications and mixed furnace number.
Especially because a mixed furnace number is very common, which causes a lot of false 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 for the high or low content of carbon.
W6Mo5Cr4V2Co5 belongs to the HSS-E type, because the carbon content is lower than the standard lower limit.
After heat treatment, the hardness does not reach 67HRC, why is it also called high-performance HSS?
Since they belong to the HSS-E type, steel mills must ensure that the steel can reach more than 67HRC.
As for the tool with or without such a high hardness, it is a tool factory internal matter, nothing to do with the steel mill, but not up to 67HRC is the steel mill’s fault.
There are also many cases of unqualified die steel composition, and disputes continue.
14. Raw material decarbonization
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 hidden quality problems.
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 and retained austenite.
Figure 20 shows the decarburization of M2 steel.
Figure 21 shows the decarburization of T12 steel (etched in 4% HNO3 alcohol solution), the fully decarburized layer is ferrite, the transition zone is carbon-lean tempered martensite, and the non-decarburized zone is tempered martensite and carbide.
Figure 19 Austempered decarburization layer (250×)
Figure 20 Decarburization of M2 steel
Figure 21 Decarburized layer of T12A steel (after quenching→tempering) (200×)
15. W18 steel with no obvious heat treatment effect
We select a W18 steel 13mm × 4.5mm flat steel from a certain company, and quench it in a salt bath at 1210℃, 1230℃, and 1270℃.
The heating time is 200s and the grain size is 10.5, as shown in Figure 22.
The hardness after quenching is between 65 and 65.5HRC.
The hardness does not increase but decreases after tempering for three times at 550℃.
This question is very strange, so it is called “an anecdote.”
Figure 22 W18 steel quenching Grade 10.5 (500×)
It’s the carbide that’s playing tricks on us, meaning that when carbide is heated, it does not dissolve into austenite, nor does it precipitate during the tempering process.
It’s simply called can’t get in or out, so where’s the secondary hardening?
The root of the anecdote is that the carbide is teasing us, which means that when the carbide is heated, it does not dissolve into the austenite, and there is no precipitation in the tempering process.
It’s simply called that “can’t get in or out”, so where’s the secondary hardening?
16. Surface Quality
Surface defects are visible to the naked eye:
The contract has to be dimensioned;
The actual supply varies in length and size;
Surface defects such as ultra-thin steel surface pits, corrosion pitting, roundness, horseshoes, excessive steel plate unevenness, and uneven thickness.
There are still many examples of steel defects.
I hope everyone will pay attention to choosing materials.
Can poor materials make good tools? Of course not!
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