Steel defects refer to various abnormal occurrences on the surface or interior of steel during its production or use that can impact its performance and quality.
Common surface defects in steel include cracks, scratches, bends, ears, scabs (heavy skin), welding scars, and edge burrs. In addition, there are typical surface defects such as bearing oxides, stains, cracks, pitted surfaces and inclusions.
The causes of steel defects are diverse, such as severe damage or wear of the previous hole-type rolling groove, foreign metals falling into the rolled parts and being pressed onto the steel surface, or defects on the surface of the previous rolled part. The oxidative atmosphere during heating also leads to the oxidation of the steel, forming oxides such as FeO, Fe2O3, Fe3O4 on the surface of the part.
Steel defect detection techniques are mainly divided into traditional manual visual detection and computer vision-based automated detection. In recent years, deep learning-based methods such as YOLOv5 and YOLOv7 have been widely applied in the automated detection of surface defects of steel.
For certain specific defects, such as banding, they can be eliminated through the high-temperature diffusion annealing method. This process involves heating above 1050℃ to allow uniform diffusion of the carbon atom, thus eliminating banding.
Steel defects not only affect the physical properties of the steel, but can also present safety risks during use. Therefore, detecting and treating steel defects is crucial to ensuring steel quality and safe use.
What are the specific reasons and mechanisms of steel defects?
The specific reasons and mechanisms for steel defects mainly include the following points:
Surface defects: These defects include cracks, scratches, bends, ears, etc. Crack formation can be due to underground bubbles in the steel ingot, uncleaned cracks and non-metallic inclusions that rupture or extend during rolling, as well as internal cracks in the steel ingot that expand and are exposed to the surface during rolling. lamination. In addition, factors such as inconsistent cooling conditions on both sides of the steel plate, uneven temperature of the rolled part, uneven deformation during the rolling process, and uneven cooling of water sprayed on the roller path of the steel belt can also cause surface defects. .
Internal defects: This includes shrinkage residues, delamination, white spots, segregation, non-metallic inclusions, looseness, etc. These defects are mainly caused by equipment, process and operational reasons during the steel manufacturing process.
Shape and size defects: These defects may involve size control issues during steel production. Although the specific generation mechanism is not detailed in the information I looked for, it can be inferred that it is related to temperature control, pressure distribution and other factors during the production process.
Other factors: For example, deficiencies caused by equipment, processes and operational reasons during carbon steel casting and rolling (forging) processing, including scales, non-metallic inclusions, etc. Technology in steel production can also cause various types of defects on the surface, such as lamination scales, stains, etc.
Types of Steel Defects
Materials form the basis for producing durable tools. During actual production, various types of material defects are often found.
Today we will enlighten you about the 16 types of steel defects so that you can be careful when selecting the raw material.
01. Porosity of raw materials
After carrying out an acid etching test on the steel, it was discovered that some regions of the sample's surface were not dense and had visible voids.
These voids, which appear as dark patches with uneven color tones compared to other areas, are known as porosity.
When the porosity is concentrated in the central part of the sample, it is called central porosity, while if it is evenly distributed on the surface, it is called general porosity.
Both GB/T9943-2008 for high-speed tool steel and GB/T1299-2014 for tool steel have specific regulations regarding steel porosity, but supplies often exceed the standard.
Porosity has a significant impact on the strength of steel and its main dangers are as follows:
- Porosity significantly weakens steel, making it susceptible to cracking during hot working processes such as forging, and also during heat treatment.
- Porosity results in tools that wear easily and have an uneven surface finish.
Because porosity affects the performance of steel, tool steel has stringent requirements for allowable porosity levels.
Figures 1 and 2 represent φ90 mm W18Cr4V steel raw materials (abbreviated as W18), showing porosity patterns and porosity cracks after a heat attack treatment with 1:1 HCl.
Figure 3 shows an image of a W18Cr4V steel slotted cutter that has suffered severe cracking due to preservation during heat treatment, as represented through heat attack with 1:1 HCl.
Figure 1 Central porosity
Figure 2 Cracking of centrally porous steel during billet forging
Figure 3 Cracks in the cutter material due to porosity during heat treatment
02. Shrinkage residue
During the casting of an ingot, the liquid steel condenses and contracts in the central part, forming a tubular hole known as shrinkage.
Typically, shrinkage is found near the feeder at the ingot head and must be removed during billet formation.
However, the portion that cannot be completely removed is called the shrinkage residue.
While it is ideal to completely remove shrinkage, steel mills often prioritize production efficiency and leave waste, resulting in irreversible consequences for subsequent processes.
Figure 4 shows φ70mm W18 steel with shrinkage residue and severe porosity, as represented by thermal attack with 1:1 HCl.
Figure 5 shows φ70 mm W18 steel with shrinkage residue that formed cracks after rolling, as represented by thermal etching with 1:1 HCl.
A few years ago, a company found shrinkage residue when sawing φ75mm M2 steel.
Figure 4
Figure 5: Cracks caused by shrinkage of W18 steel
03. Surface crack
Longitudinal cracks on the surface of high-speed steel raw materials are a common occurrence.
There can be several causes for this, such as:
(1) During hot rolling, stress concentration may occur during the cooling process, leading to cracks along the score lines due to incomplete removal of surface cracks or scratches caused by holes in the die.
(2) Poor quality die holes or large feed rates during hot rolling can lead to bending, which causes cracks along the bend lines in subsequent processing.
(3) Cracks may be produced during hot rolling if the rolling stop temperature is too low or the cooling rate is too fast.
(4) Surface cracks are often observed in 13 mm × 4.5 mm W18 flat steel that is rolled in cold winter, indicating that cracks can also be influenced by weather conditions.
However, no cracks are observed when the same type and specification of steel is rolled at other times.
Figure 6 shows the surface crack of φ30mm W18 steel, with a depth of 6mm, as represented by thermal attack with HCl 1:1.
Figure 6 Surface crack
04. Cracks in the center of the raw material
During the hot rolling process of high speed steel, excessive deformation can cause the core temperature to increase instead of decreasing. This can lead to cracks forming in the center of the material due to thermal stress.
Figure 7 shows the central crack in φ35mm W18 steel (etched with HCl 1:1).
Core cracks in high-speed steel raw materials are common in tool factories, but are harmful because they are invisible and cannot be detected by touch. The only way to observe these cracks is through flaw detection.
Figure 7 Central fissure
05. Segregation
The unequal distribution of chemical elements within an alloy during the solidification process is known as segregation. This can have a significant impact on the performance of the steel, especially if there is an uneven distribution of impurities such as carbon.
Segregation can be divided into microsegregation, density segregation and regional segregation.
Density segregation occurs due to differences in the density of the alloy's constituent phases, causing heavier elements to sink and lighter elements to float during solidification. Regional segregation is caused by the local accumulation of impurities in ingots or castings.
Figure 8 shows a hardened metallographic sample of W18 steel (etched with 4% HNO3 alcohol solution), which reveals a cross-shaped pattern.
Further analysis of the chemical composition showed that the matrix part had a lower carbon content, while the cross-shaped part had a higher carbon content.
This cross shape is a result of square segregation caused by the segregation of carbon and alloy components during the rolling process.
Severe regional segregation can weaken the strength of the steel and make it more susceptible to cracking during hot working.
Figure 8 Cross-shaped segregation (3×)
06. Non-uniformity of carbide
The extent to which the eutectic carbides in high-speed steel (HSS) break down during the hot pressing process is called carbide non-uniformity. The greater the deformation, the greater the degree of fracture of the carbide and the lower the level of non-uniformity of the carbide.
When the carbides in steel are severely broken down, such as in the form of thick ribbons, meshes or large accumulations of carbide, this has a significant impact on the quality of the steel. Therefore, it is crucial to carefully control the non-uniformity of carbide to ensure the quality of HSS tools.
Figure 9 shows the effect of carbide non-uniformity on the flexural strength of W18 steel.
As can be seen from the figure, the flexural strength in grades 7-8 with non-uniformity is only 40-50% of grades 1-2, reducing the strength to 1200-1500MPa, which is only equivalent to the grade level of higher toughness in cemented carbides. Horizontal performance is about 85% of vertical performance.
The concentration and band-like distribution of carbides can also result in uneven tempered grains and uneven dissolution of carbides, leading to an increased tendency to overheat and a reduction in secondary hardening capacity, respectively.
Figure 9 shows the impact of carbide non-uniformity on the flexural strength of W18Cr4V high-speed steel.
It can be seen that severe non-uniformity of carbide can result in cracking and overheating during hot working, causing the finished tool to fail during use.
Figure 10 illustrates the quenching crack caused by coarse zonal carbides in W18 steel (etched with 4% HNO3 alcohol solution).
Figure 10 Coarse zonal carbide
07. Network carbide
Steel that has undergone hot rolling or annealing can form network carbides due to high heating temperatures, prolonged retention times that cause grain growth, and slow cooling processes that result in carbide precipitation along grain boundaries. grains.
The presence of cross-linked carbides greatly increases the fragility of the tool, making it more prone to chipping. In general, full lattice carbides are not acceptable in steel.
Inspection for network carbides must be performed after quenching and tempering.
Figure 11 shows the cross-linked carbides of T12A steel (attacked with 4% HNO3 alcohol solution), while Figure 12 shows the morphology of the cross-linked carbides of 9SiCr steel (attacked with 4% HNO3 alcohol solution), revealing severe overheating during annealing process.
Figure 11 T12A Steel Mesh Carbide (500×)
Figure 12 9SiCr Steel Mesh Carbide (500×)
08. Hardened carbide putty
Tool cutters performing HSS turning or milling may encounter a hard substance and suffer damage. This defect is normally not easily found during high-speed turning due to the high cutting speed and noise.
However, during milling, lumps and strange chaos may be observed, such as a squeaking sound and severe tool wear when milling channels with twist drills.
Upon inspection, the bright blocks can be seen with the naked eye and have extremely high hardness, reaching 1225HV, while the non-hard areas are in a normal state of annealing. This is known as “hardened dough”.
The presence of hardened masses results in damage to the tool and makes cutting difficult.
The formation of these hard lumps is believed to be caused by the segregation of chemical components during the casting process and may be a type of high hardness composite carbide or the result of the addition of refractory alloy blocks during casting.
Figure 13 shows the macrostructure of a hardened mass in W18 steel (attack with 4% HNO3 alcohol solution), with the white substance being the hardened mass and the gray and black areas representing the drill grooves.
Figure 13 The macrostructure of the hardened mass of W18 steel (20×)
09. Inclusions
Inclusions are a common defect in steel that can be classified into two categories: metallic inclusions and non-metallic inclusions.
Metallic inclusions form due to incomplete melting of the ferroalloy during the casting process or the presence of foreign metallic particles remaining in the steel ingot.
Non-metallic inclusions are divided into two types:
(1) endogenous inclusions, mainly caused by dirty casting systems, removal of refractory mud from equipment, or use of unclean charge materials;
(2) inclusions produced and precipitated due to chemical reactions during the casting process. Figure 14 shows metallic inclusions found in W18 steel, while Figure 15 shows non-metallic inclusions causing cracks during quenching (attack with 4% HNO3 alcohol solution).
Figure 14 Metallic inclusions
Figure 15 Cracks caused by non-metallic inclusions during quenching (400 x)
Inclusions are detrimental to the quality of the steel. They segment the steel matrix, decrease its plasticity and strength, making the steel susceptible to cracking around inclusions during rolling, forging and heat treatment.
Inclusions can also cause fatigue in the steel, as well as difficulties during cutting and grinding. Therefore, tool steel must have specific requirements for inclusions.
10. Bulk carbide
In the steel casting process, uneven distribution of carbides may occur due to segregation of components or when carbides in the iron alloy are not fully melted, resulting in large angular carbides that persist without being crushed after forging.
The presence of these carbides in bulk increases the fragility of the tool and increases the risk of tipping.
During the heat treatment process, these large carbides and alloying elements can become enriched, potentially leading to defects such as overheating, insufficient tempering, and even cracking along grain boundaries.
Figure 16 shows superheating during quenching caused by segregation from surrounding large carbide components (etched in 4% HNO3 alcohol solution).
Figure 16 Overheating caused by segregation of components around bulk carbides during quenching (500×)
11. Carbide settlement
In the liquid metal solidification process, the segregation of carbon and alloy elements can cause large blocks of carbide to precipitate during cooling.
This segregation, known as liquation, is not easily eliminated during subsequent processing and results in the presence of bulk zoster carbide in the rolling direction of the steel.
Figure 17 shows the liquefaction of CrMn, recorded with a 4% HNO3 alcohol solution.
Figure 17 Carbide Settlement (500×)
Liquefaction steels are highly brittle as the continuous metal matrix is disrupted, resulting in reduced strength. Previously, settlement was commonly found in CrWMn and CrMn steels, and using them to make gauges often resulted in difficulty in obtaining a smooth surface.
12. Carbon graphite
Because the annealing temperature is very high and the retention time is very long, during the slow cooling process of the steel, the carbides easily decompose into free carbon, known as graphite.
Figure 18 shows the microstructure of graphite carbon in T12A steel (attacked with 4% bitter acid alcohol solution).
Figure 18 Graphitic carbon microstructure of T12A steel (500×)
The precipitation of graphite carbon significantly decreases the strength and toughness of the steel, making it unsuitable for the production of knives and critical components. Steel shows black fractures when it contains high levels of graphite carbon.
The presence of graphite carbon can be determined through chemical analysis for both qualitative and quantitative analyses, and its shape and distribution can be observed using metallographic methods.
Additionally, there will be an increase in the ferrite fabric around the graphite.
13. Failure in mixing and composition
Material mixing in tool and mold manufacturing companies is a common problem, the result of poor management and a low-level defect. Mixed materials can include three aspects: mixed steel, mixed specifications and mixed furnace numbers.
The latter is especially prevalent and can cause a lot of problems with false heat treatments without recourse. From time to time, unqualified tool material components are also found.
Some high-speed steel components do not meet the GB/T9943-2008 high-speed tool steel standard, especially regarding high or low carbon content. For example, W6Mo5Cr4V2Co5 belongs to the HSS-E type, but has a carbon content lower than the standard lower limit.
Despite being labeled as high performance HSS, after heat treatment the hardness does not reach 67HRC. Steel mills must ensure that the steel can reach a hardness of at least 67HRC if it belongs to the HSS-E type.
Whether a tool requires such a high hardness is an internal matter for the tool factory and is not the responsibility of the steel company.
However, if the hardness does not reach 67HRC, it is the steel company's fault. There are also many cases of unqualified composition of matrix steel, leading to continuous disputes.
14. Decarbonization of raw materials
The country has established standards for the decarburization of steel, however, steel suppliers often supply materials that exceed these standards, resulting in significant economic losses for tool manufacturing companies.
The surface hardness of tools decreases and their wear resistance is low after quenching for materials with a decarburized layer. Therefore, it is necessary to completely remove the decarburized layer during machining to avoid possible quality problems.
Figure 19 illustrates the decarburization morphology of W18 steel raw material (etched in 4% HNO3 alcohol solution). The decarburization zone is needle-shaped quenched martensite, while the non-decarburization zone is composed of quenched martensite, carbides, and retained austenite.
Figures 20 and 21 show the decarburization of M2 and T12 steels, respectively (etched in 4% HNO3 alcohol solution).
In the case of T12 steel, the fully decarburized layer is ferrite, the transition zone is composed of low-carbon tempered martensite and the non-decarburized zone is composed of tempered martensite and carbides.
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 without obvious heat treatment effect
We selected a flat bar of W18 steel with dimensions of 13mm x 4.5mm from a specific company and tempered it in a salt bath at temperatures of 1210°C, 1230°C and 1270°C.
The heating time was 200 seconds and the grain size was 10.5, as shown in Figure 22. The hardness after quenching was between 65 and 65.5HRC, but surprisingly, the hardness decreased after tempering at 550° C three times.
This anomaly is referred to as “a joke.
Figure 22 Steel tempering W18 Grade 10.5 (500×)
It appears that the carbide is playing a trick on us, which means that when the carbide is heated, it does not dissolve into austenite or precipitate during the tempering process.
This is simply referred to as not being able to get in or out, so where is the secondary hardening?
The root of the problem is that the carbide is triggering us, which means it does not dissolve into the austenite during heating and there is no precipitation during the tempering process.
It's simply a case of not being able to get in or out, so where does the secondary hardening come from?
16. Surface Quality
Surface defects are easily visible to the naked eye, such as:
- Inconsistent dimensions in the contract;
- Length and size deviation from the actual offer;
- Surface imperfections including ultra-thin steel surface pits, corrosion pits, roundness problems, horseshoes, excessive irregularities in the steel plate and uneven thickness.
What are the specific impacts of steel defects on the physical properties of steel?
The specific impacts of steel defects on the physical properties of steel mainly include the following aspects:
Changes in hardness and plasticity: Influenced by certain factors, the strength of steel can increase, but at the same time, plasticity and toughness decrease, resulting in increased brittleness, a phenomenon known as hardening. This typically occurs under repeated loads when the elastic limit increases and enters the plastic stage.
Effects on wear and fatigue resistance: Surface quality defects not only affect the aesthetic appearance of hot-rolled steel strips, but can also have adverse effects on their mechanical properties and corrosion resistance, including wear and fatigue resistance .
Tool wear and uneven surfaces: The presence of looseness in the material can lead to excessive wear and uneven surfaces of tools made from it. Consequently, tool steel has strict requirements for the acceptable level of looseness.
Dispersion of microstructure and defects: The toughness of steel depends mainly on the dispersion of microstructure and defects (avoiding concentrated defects), and not on the chemical composition. Toughness undergoes significant changes after heat treatment.
Effects of annealing and normalizing treatment: Annealing can reduce the hardness of steel, improve plasticity, refine grain, eliminate structural defects caused by casting, forging and welding, homogenize the structure and composition of steel, and relieve internal stresses and work hardening of steel. Normalizing has similar effects on large castings, forgings, and welding.
