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: Decarburization
There are only a handful of things to check when your steel parts fail to respond to quench and temper heat treatment.
Here’s my list in decreasing probability order:
- Mixed steel / wrong steel being heat treated.
- Decarburization on the surface.
- Failure to heat it above the austenitizing temperature.
- Failure to hold it for sufficient soaking time. (This can be especially problematic with induction processes)
- Failure to quench fast enough.
So someone could say “Well you didn’t mention that the steel had a microstructure that interfered with the process and is preventing us from getting the hardness required.”
To them I would say “Please see items 3 and 4 above.”
Photo credit Da Wei Induction Heating Machine Inc.
Quench cracks result from stresses produced during the transition from Austenite to Martensite, which involves an increase in volume.
The martensitic transformation starts at the outermost surfaces of the part being quenched. As the transformation goes deeper into the softer austenite towards center of mass, its change in volume is restricted by the martensite already created in the outer volumes of the part adjacent to the surface.
This creates internal stresses which place the surface into tension.
When enough martensite has formed to create internal stress greater than the ultimate strength (tensile strength) of the as quenched martensite at the surface, a crack results.
As-quenched Martensite is hard and brittle- it has virtually no ductility.
Here are 3 ways to recognize a quench crack:
1) The crack runs from the surface towards the center of mass in a fairly straight line. The crack will also tend to be open or spread at the OD surface.
2) Quench cracks do not have decarburization apparent, since the quenching occurs at relatively low temperatures. If there is decarb associated with a crack, that shows that the crack existed at the time the material was at temperatures hot enough to decarburize. In other words, the crack existed prior to austenitizing.
3) The fracture surfaces will exhibit a fine crystalline structure. I remember the first time I saw a quench crack, thinking, “it crystallized.” Well, the steel is already crystalline, but the fine martensitic structure revealed by the crack showed that there was absolutely no ductility in the material…
Bonus tip: if you see a build up of scale in the crack itself, that tells you that the crack was there after quenching but before tempering. During the tempering operation at tempering temperature, oxygen in the atmosphere created a scale where it could reach the iron in the crack.
For more information on Quench Cracks, look at our blog posts Here and Here.
“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.
Failures of steel parts in service or production occur very infrequently. However, when steel parts fail, the consequences are dire.
Here are 7 ways that steel can fail as a result of Quench Cracking from heat treatment.
- Overheating during the austenitizing portion of the heat treatment cycle can coarsen normally fine grained steels. coarse grained steels increase hardening depth and are more prone to quench cracking than fine grain steels. Avoid over heating and overly long dwell times while austenitizing.
- Improper quenchant. Yes, water, brine, or caustic will get the steel “harder.” If the steel is an oil hardening steel, the use of these overly aggressive quenchants will lead to cracking.
- Improper selection of steel for the process.
- Too much time between the quenching and the tempering of the heat treated parts. A common misconception is that quench cracks can occur only while the piece is being quenched. This is not true. If the work is not tempered right away, quench cracks can (and will) occur.
- Improper design– Sharp changes of section, lack of radii, holes, sharp keyways, unbalanced sectional mass, and other stress risers.
- Improper entry of the part/ delivery of the quenchant to the part. Differences in cooling rates can be created, for example, if parts are massed together in a basket resulting in the parts along the edges cooling faster than those in the mass in the center. Part geometry can also interfere with quenchant delivery and effectiveness, especially on induction lines.
- Failure to take sufficient stock removal from the original part during machining. This can leave remnants of seams or other surface imperfections which can act as a nucleation site for a quench crack.
Finally, materials that are heat treated to very high strength levels, even though they did not quench crack, may contain localized concentrations of high residual stresses. If these stresses are acting in the same direction as the load applied in service, an instantaneous failure can occur. This will be virtually indistinguishable from a quench crack during an examination, due to its brittle failure mode, lack of decarburization on surface of the fracture, or other forensic evidence of a process failure.
When looking at quench cracking failures under the microscope, cracks and crack tributaries that follow the prior austenitic grain boundaries are a pretty good clue that grain coarsening and or its causes- overheating or too long time at temperature- occurred. Temper scale on the fracture surface helps the metallurgist know that the crack was present before tempering. Decarburization may show that the crack was open prior to quenching.
Photo1 Thanks to WIP SAMI over at British Blades for the photo.
