Decarburization on surface layers can affect heat treatment and hardness attained on parts. Decarburization also provides evidence of where in a process a defect or imperfection occurred.
Most defects in steel workpieces encountered in our precision machine shops are longitudinal in nature. While their presence alone is enough to concern us, for the purposes of corrective action, it becomes important to identify where in the process the longitudinal imperfection first occurred. Visual examination alone is not enough to confirm the source. Did it occur prior to rolling? During rolling? After rolling? Understanding decarburization and how it presents in a sample can help us to identify where and when in the process the imperfection first occurred.
The question that we want to answer as part of our investigation is usually “When in the process did the defect first occur?” Looking at decarburization and any subscale present can help us answer that question with authority.
What is Decarburization?
“Decarburization is the loss of carbon from a surface layer of a carbon containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.”– Metals Handbook Desk Edition
The carbon and alloy steels that we machine contain carbon. In the photo above, the carbon is contained in the pearlite (darker) grains. The white grains are ferrite. In an etched sample, decarburization surrounding a defect is identified as a layer of ferrite with very little, or none of the darker pearlitic structure typically seen in the balance of the material. The black intrusion in the photo above is the mount material that has filled in the crevice of the seam defect.
What is Subscale?
Subscale is a reaction product of Oxygen from the atmosphere with various alloying elements as a result of time at high temperatures. The presence or absence of the subscale is the indicator that helps us to pinpoint the origin of the defect. For a subscale to be present, the time at temperature must be sufficient for oxygen to diffuse and react with the material within the defect. According to Felice and Repp, 2250 degrees F and fifteen minutes is necessary to develop an identifiable subscale. Lower temperatures would require longer times. Typically rolling mill reheat cycles offer plenty of time to develop a subscale in a prior existing defect. However, for defects that are created during rolling, the limited time at temperature and the decreasing temperatures on cooling make formation of subscales unlikely.
Reading Decarb and Subscale to Understand the Defect
Decarburization is time and temperature dependent. This means that its relative depth and severity are clues as to time at temperature, though interpretation requires experience and understanding of the differences in appearance from grade to grade based on Carbon content.
Symmetrical Decarburization
If the decarburization is symmetrical this is an indication that the defect was present in billet or bloom prior to reheat and rolling. oxygen in the high temperature atmosphere of the reheat furnace depletes the carbon equally from both sides of the pre-existing defect.
Asymmetrical Decarburization
Decarburization that is obviously asymmetrical indicates that the defect is mechanical in nature and was induced some time during the hot rolling process.
Ferrite Fingers
Ferrite fingers are a surface quality problem that is associated with longitudinal bar defects. During reheat, a defect in the bllom or billet is exposed to high temperature atmosphere, forming decarburization and subscale around the defect. Rolling partially closes or “welds shut” the crack. However, a trail of of subscale is entrained in a formation of almost pure ferrite which has been depleted of pearlite, carbon and alloy by the reaction at elevated temperature. This trapped scale remains a potential oxygen source, driving further internal oxidation and decarburization if temperatures remain high.
Continuous improvement requires taking root cause corrective action. Obviously identifying the root cause is critical. When we encounter longitudinal linear defects in our steel products, using a micro to characterize the nature of the decarburization and presence or absence of sub scale or ferrite fingers are important evidence as to when, where, and how in the process the defect originated.
Tag: Steel Defects
In cold worked steels, failures can be broadly categorized in two categories. The first, are those nucleated by localized defects- such as seams, pipe, and exogenous inclusions. The second, are those which result from exceeding the strength of the material itself.
The compressive stresses of cold working results in failures by shear along planes 45 degrees to the applied stress. These are known as shear failures. The presence of shear failures in an otherwise metallurgically normal material indicates excessive mechanical deformation. While often the result of tooling issues, conditions which lower material ductility including chemistry, macrostructure, nonmetallics, microstructure, aging, and hydrogen embrittlement have also been implicated in investigations of premature shear failure.
This post will focus on the central Bursts in the product of cold drawn steel, especially from the point of view of a shop making parts on automated equipment.
Ignoring the steel factors that may play a role in triggering the central bursts or chevrons, the role of tooling is usually considered to be the root cause, as replacement of dies typically eliminates the central bursting.
A bar which exhibited central bursting was saw cut lengthwise to show the internal ruptures.
In very rare cases, while machining parts from a bar which exhibits internal bursts or chevrons, the part will separate from the bar in process because of the prior existing rupture. The photo below shows such a part, note the fracture surface on the sides of the stepped down diameter on the part end shown in the photo below.
The following two photos show how the internal bursts could have been manifested in the original bar as well as the parts.
It is difficult to see the defect on the threaded end of the nearly completed part, but this photo does attempt to show that.
In a later post we will discuss more factors relating to central bursting or chevron failures of cold drawn or cold extruded steel.
Scabs are irregularly shaped, flattened protrusions caused by splash, boiling or other problems from teeming, casting, or conditioning.-AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
(Teeming refers to the process of filling an ingot mold with molten steel from the ladle. We’ll point out some continuous casting analogs later in this post.)
Scabs have scale and irregular surfaces beneath them; they tend to be round or oval shaped and concentrated to only certain blooms or billets. Scabs are always the same chemistry as the steel bloom or billet.
(If the gross irregular surface protrusion characteristic is appearing on all product, it is not likely to be a scab. If the protrusion is a different analysis, it is likely to be mill shearing.)
To differentiate between scabs and rolled in scale, scabs are ductile when bent while scale is brittle and crumbles.
If the protrusion is brittle, it may be rolled in scale.
Scabs are primarily an ingot process issue related to teeming, but we have seen them on continuous cast products as a result of mold and tundish anomalies.
Scabs present with scale beneath; Cracks may (but are not always) be present associated with the scab due to stress concentration causing the material underneath to crack. (Not the crack causing the scab…)
Ingots or blooms showing scabs should be conditioned to remove the scabs. Thermal conditioning of billets (hot scarfing or torch conditioning) can sometimes leave artifacts which present as scabs upon rolling.
While scabs can be confused with slivers, shearing, rolled in scale, or tearing, their ductility eliminates them as rolled in scale. Scabs are distinct from shearing as scabs are isolated by occurrence and have an irregular surface beneath them, while shearing usually presents as multiple instances in a single orientation along the bar. Tearing is characterized by chevron shaped breaks rather than oval shaped protrusions.
“Slivers are elongated pieces of metal attached to the base metal at one end only. They normally have been hot worked into the surface and are common to low strength grades which are easily torn, especially grades with high sulfur, lead and copper.”- AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Slivers may be caused by bar shearing against a guide or collar, incorrect entry into a closed pass, or a tear due to other mechanical causes. Slivers may also be the result of a billet defect that carries through the hot rolling process.
Slivers often originate from short rolled out point defects or defects which were not removed by conditioning.
Billet conditioning that results in fins or deep ridges have also been found to cause slivers and should be avoided. Feathering of of deep conditioning edges can help to alleviate their occurrence.
Slivers often appeared on mills operating at higher rolling speeds.
When the frequency and severity of sliver occurrence varies between heats, grades, or orders, that is a clue that the slivers probably did not originate in the mill.
Slivers are often mistaken for shearing, scabs, and laps. We will post about these other defects in the future.
“Laps are longitudinal crevices at least 30 degrees off radial, created by folding over, but not welding material during hot working (rolling). A longitudinal discontinuity in the bar may exist prior to folding over but the defect generally is developed at the mill.”- AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
In plain language, a lap is a ‘rolled over condition in a bar where a sharp over fill or fin has been formed and subsequently rolled back into the bar’s surface.’
An etch of the full section shows what is going on in the mill. Laps were often related to poor section quality on incoming billets, although overfill scratches, conditioning gouges from “chipping” have also been shown to cause laps.
Laps are often confused with slivers, and mill shearing which we shall describe and post soon.
The term ‘lap seam’ is sometimes used, but it is careless usage; it implies the lap is caused by a seam – it is not; a seam is a longitudinally oriented imperfection, and so is used in this mongrel term as a shorthand way of saying ‘longitudinal.’
Modern speakers sometimes try to use the word ‘lamination’ to describe laps but as we will see, not all lamination type imperfections are laps…