Steel Quality Data
Defects and Discontinuities in Wrought Steel Bar
Rolled and forged bar carries imperfections from the ingot, from the reduction that followed, and from every process applied after forming. This page collects the flaw types the source illustrates on rolled bar, the ingot conditions behind them, and the rules that decide when a discontinuity is a defect and when it is not.
A discontinuity is not automatically a defect Manufactured material always contains imperfections, and the source is direct about the consequence. A discontinuity becomes a defect when it interferes with the function and the expected life of the part. Until then it is a feature of the material that a drawing, a standard or an agreed acceptance level has to place limits on.
- The distinction matters in both directions. A notch, a seam or an inclusion that sits on a fracture surface is not proof that it caused the failure, and an investigation that stops at the first imperfection it finds can send the corrective action to the wrong place. What a fracture surface can and cannot establish is set out on the fracture surface features chart.
- Where a crack starts is partly predictable from the load case, and the expected origin moves with the geometry. In an unnotched bar under plain tension the centreline is the likely site. Under three point bending it is the point of maximum bending moment. Under rolling contact it sits below the surface, where the maximum stress develops. Where the fracture starts somewhere else, the implication is that a geometric or material imperfection moved the local maximum stress.
- Those rules are what let a buyer or a tool room read a delivered bar rather than only a failed one. The checks that a shipment passes before it leaves are set out on the quality verification page.
The flaw types found in rolled bar The source illustrates ten types of flaw on a rolled bar section. The list below is that set, with what each one is. Terms for these flaws vary between industries and locations, so the name on a rejection note is worth checking against the shape on the bar.
| Flaw | What it is |
|---|---|
| Inclusions | Nonmetallic particles carried from melting and casting, stretched out in the working direction. Singled out in the source as one of the most common imperfections involved in failures. |
| Laminations from spatter | Splashes of metal that froze on the mould wall during pouring and were later rolled into flat separations in the product. |
| Slivers | Loose or torn pieces of steel that were rolled into the surface. |
| Scabs | Metal splashed against the mould wall, oxidised where it stuck, and usually visible only after rolling, as a poor surface finish. |
| Pits and blisters | Gaseous pockets trapped in the ingot. They can remain as surface or near surface defects after working. |
| Embedded scale | Scale formed during an earlier heating operation and then rolled into the surface instead of being removed. |
| Cracks | Formed as the metal cools in the mould, with little or no oxide on their edges, and they leave highly stressed areas in the product. |
| Seams | Linear discontinuities that develop from elongated trapped gas pockets or from cracks during working. A seam is often the visible form that an ingot surface defect takes after rolling. |
| Laps | Hot metal folded over and worked into the surface without bonding, because oxide sits between the two surfaces. The result is a sharp discontinuity. |
| Chevron or internal bursts | Internal tears, invisible from the outside, which makes them one of the more dangerous defects to find in a finished bar. |
Source: ASM Handbook, Vol. 11, Failure Analysis and Prevention (ASM International), the ten flaw types in rolled bar shown in the article Failures Related to Metalworking. The source notes that the set is not complete, for example die scratches on cold worked product are not included.
Where the flaws come from Most of these discontinuities can be traced to the pouring and solidification of the ingot, and the rest to the reduction that follows. The six families below are the ones the source traces to the original ingot.
| Source condition | What it is | How it is found |
|---|---|---|
| Chemical segregation | Alloying elements and impurities are never distributed evenly in a casting. Microsegregation sits in the interdendritic regions over a few to several hundred microns, and can be removed by homogenisation. Macrosegregation runs from the surface to the centre, gets worse with section size, and is not economic to remove. Hot working breaks up the cast structure and needs a minimum amount of reduction, but a badly segregated ingot only turns its segregation into banding. Figure 3 of the source shows a carbon rich centreline in a rolled 1041 bar, and Figure 4 shows ferrite and pearlite bands in a plain carbon 1022 steel. | Macroetching shows banding plainly on a transverse or longitudinal section. Banded structure can be seen in the finished part as well, and it changes how the steel responds to heat treatment and to loading across the banding direction. |
| Ingot pipe, porosity and centreline shrinkage | Steel shrinks about 5 percent by volume as it solidifies, gases trapped by the freezing metal cannot all escape, and small crevices in the mould wall tear the metal as it is stripped. The result is pipe and blow holes. Whether they survive into the product depends on the reduction ratio and on how much healing the conversion practice delivers. | Macroetching and ultrasonic inspection are the two methods the source names as the most widely used for identifying regions of unsoundness. This is where an ultrasonic acceptance level on a purchase order does its work. |
| High hydrogen content | Hydrogen dissolved in the liquid steel comes out of solution as the ingot solidifies and can leave internal cracks. In tool steels and some medium carbon low alloy grades the result is flaking, and vacuum degassing is the usual way to bring the hydrogen down to a level that does not flake. | Flakes sit away from the surface, in the centre of the section, which is why they are found on a macroetched section rather than by looking at the bar. The embrittlement mechanisms that hydrogen drives are collected on the steel embrittlement types page. |
| Nonmetallic inclusions | Inevitable in commercial alloys. Some are entrapped foreign matter such as refractory lining, and some form inside the metal when temperature or composition changes, which is why conventional melting cannot remove them entirely. | Macroetching gives a good indication of cleanliness. The pit that an inclusion leaves must be told apart from the pit left by a metallic segregation, and the test is a comparison of an annealed specimen with a hardened and tempered one etched the same way. |
| Unmelted electrodes and shelf | Listed in the source among the imperfections that can be traced back to the ingot, alongside the others above. | The same sectioning and inspection routes apply. A macroetched section is cheap compared with the cost of finding it in a finished tool. |
| Cracks, laminations, seams, pits, blisters and scabs | The surface and near surface group, formed at the mould wall and during the first reduction. Their common feature is that they were open to the atmosphere while the metal was hot. | Oxide in the defect is the evidence that it was open while hot. A hot alkaline chromate etch brings out the oxygen enrichment that proves it. |
Source: ASM Handbook, Vol. 11, the imperfections from the ingot section of the same article.
Nonmetallic inclusions in more detail Inclusions deserves their own section because they are the most common imperfection in failure work and the one a buyer can influence most directly on a purchase order.
- Two categories are recognised. The first is entrapped foreign matter that finds its way into the metal while it is molten or being cast, such as refractory lining. The second forms inside the metal when temperature or composition changes, and separates out in the liquid or on solidifying. In steels, aluminates and silicates generally form before solidification, while sulfides form during it, with manganese sulfide settling in the interdendritic regions and primary grain boundaries where the last liquid freezes.
- An inclusion is a stress concentrator because it is discontinuous with the surrounding metal and its properties do not match. Brittle inclusions that fracture and fragment during working are the worst of them, and fragmentation is common with certain refractory and slag inclusions when the cross sectional reduction is large.
- Deformability decides how much damage an inclusion does downstream. Manganese sulfide is very deformable at hot working temperature, so it elongates along the working direction and makes the material anisotropic, with measurable losses in transverse ductility, fatigue life and fracture toughness. Inclusion shape control aims at the opposite result, using a chemistry modification such as a calcium or rare earth treatment so the inclusions stay globular instead of stringing out. Spherical inclusions are less of a stress concentrator, and the source cites fatigue and Charpy data showing the improvement.
- The deleterious nature of an inclusion depends on its composition, its volume fraction, its shape, its orientation and its properties relative to the matrix. A hard particle under load concentrates stress locally by an amount that depends on the elastic moduli, the size, the shape and the orientation, which is why the same inclusion content can be tolerable in one part and not in another.
- Reading cleanliness from an etch needs care. Inclusions show up as pits or pinholes after macroetching, and those pits have to be separated from pits left by metallic segregation or by the etching procedure itself. The source gives the test, an annealed specimen and a hardened and tempered specimen etched the same way. Inclusion pits look similar in both. Segregation pits are more prominent in the hardened specimen.
Surface flaws from the reduction The first working of the ingot or billet adds its own defects, some of them related to the casting flaws above and some produced by the working itself.
| Flaw | What it is |
|---|---|
| Laps | Linear defects from the folding over of hot metal at the surface, worked in but not bonded because oxide sits between the surfaces. The result is a sharp discontinuity with no metallurgical connection across it. |
| Seams | Linear discontinuities that start as a crack, as a heavy cluster of nonmetallic inclusions, or as a deep lap. A surface hole in the ingot that oxidises and cannot heal simply stretches during rolling into a crack like seam. |
| Slivers and rolled in scale | Loose steel and scale worked into the surface during reduction. |
| Ferrite fingers | Surface cracks that were welded shut again but still contain the oxides and the decarburisation of the crack. The defect is closed yet its consequences stay in the metal. |
| Fins and overfills | Protrusions formed by incorrect reduction during hot working. |
| Underfills | The result of incomplete working of the section during reduction. |
| Internal bursts | Internal tears where the work metal is weak, from pipe, porosity, segregation or inclusions, and the tensile stresses of working are high enough to pull it apart. If segregation has left low melting phases in the metal, those phases can cause bursts as well. An excessive hot working temperature makes the whole picture worse. |
Source: ASM Handbook, Vol. 11, surface flaws from preliminary reduction in the same article. Excessive hot working temperature helps produce bursts and several of the others.
- Bursts and the other internal tears are the reason a forging or a heavy bar gets an ultrasonic check before expensive machining starts. External flaws are visible on a dressed surface, internal ones are not, and the two need different inspection routes.
- The forging temperature window a grade is worked in is set out on the tool steel forging temperature guide, and the defects a hot working operation itself can add, including laps and bursts, are part of what a forging procedure is written to avoid.
Defects added after forming Bar that leaves the mill sound can still be damaged by the processes applied to it afterwards. The source lists the operations and the defects each one can introduce, and the list reads as a checklist for a route card.
| Proceso | Defects it can introduce |
|---|---|
| Electroplating | Hydrogen embrittlement, galvanic corrosion. |
| Tratamiento térmico | Excessive grain growth, burning of grain boundaries, a brittle structure, carburisation, decarburisation, quench cracks. |
| Electrolytic cleaning | Pitting. |
| Surface hardening, nitriding, carburising, anodic hard coating | Excessive case thickness, microcracks, embrittled material at stress raisers. |
| Mecanizado | Tool marks, grinding cracks. |
| Soldadura | Weld metal defects, hydrogen induced cracking, inclusions, improper structure. |
Source: ASM Handbook, Vol. 11, defects that may result from postforming processes.
- Two of the entries are the ones a tool steel buyer meets most often. Heat treatment can leave a decarburised skin or a carburised one, and either changes how the tool performs at its surface. Machining can leave tool marks and grinding cracks, both of which act as stress raisers under load.
- Decarburisation is bought and sold as a depth, and the standard machining allowance on mill products exists partly to remove it. The allowances by section are tabulated on the tabla de tolerancias de mecanizado.
- The failure patterns these postforming defects produce in service, and the way each one is recognised, are worked through case by case on the tool and die failure analysis page.
Reduction ratio and the sound centre
Casting flaws are healed or carried forward by the amount of hot work the material receives. Large sections that get limited reduction are where an unsound centre survives, which is why the question is asked of bar and billet rather than of finished tools.
What to write into a purchase order
Grade, condition, size and tolerance are the visible part. Surface condition, decarburisation depth, an ultrasonic acceptance level and a certificate requirement are the parts that decide whether an internal discontinuity is your problem or the mill’s.
When a spall or a chip follows a defect
Impact loaded edges fail differently from a straight overload, and some spalls start at a crack that was already in the tool. The mechanisms, coefficients and hardness limits are collected on the impact wear and spalling page.
When the defect was there before service
Scale or oxide in a crack proves the crack was open while the metal was hot, which dates it to the mill or to the forge rather than to the press. A crack that opened during the quench has its own signature, set out on the página de agrietamiento de enfriamiento.
Confirm before quoting
This page describes general steelmaking and working practice for reference. Acceptance limits for a delivered bar come from the standard or the drawing that governs your own order, and they should be agreed before production rather than argued after delivery.
A discontinuity on a failed part is not by itself a root cause. Where the question is whether a defect caused a failure, the answer comes from analysis of the part, its processing record and its service history.
Páginas de referencia relacionadas
Guía de tratamiento térmico del acero para herramientas · ESR tool steel · Hardness testing methods · Cold work tool steels · Hot work tool steels · Tool steel supply
Source: ASM Handbook, Volume 11, Failure Analysis and Prevention (ASM International), article Failures Related to Metalworking, the imperfections in wrought forms, imperfections from the ingot, nonmetallic inclusions, forging imperfections and postforming defect sections. Reference data for general understanding only. Confirm the acceptance criteria for your own order against the governing standard. Aobo Steel supplies tool steel in the annealed condition.