The subject of carbide formation in steels has intrigued The Doctor ever since he first peered into a microscope and observed them (Fig. 1). It is also a very important subject for the heat treater to better understand. Carbides can be our friend or our enemy. Let’s learn more.

Carbide Formation

In simplest terms, alloying elements in steels can be divided into two groups: those that do not form carbides (e.g., Al, Co, Cu, N, Ni and Si) and those that do (e.g., Mn, Cr, W, Mo, V, Zr, Nb, Ta and Ti). This latter group is arranged in accordance with their affinity for carbon.

The periodic table of elements (Fig. 2) tells us these carbide formers fall to the left of iron. Unstable carbides, those that will dissociate on heating, can be found at the far left end of each row, while the elements closest to iron form extremely stable carbides that dissociate at temperatures much higher than the critical temperatures for steel.[1]

Carbide formation in steels is typically limited to a few carbide types (Table 1). Here M represents the carbon-forming elements in steel. The types, combinations and amount of alloying elements present complicate carbide formation. For example, in a Cr-Mn steel, the carbide (Cr,Mn,Fe)23C forms in lieu of a Cr23C carbide.[1]

The stability of the carbides is highly dependent on the presence of other alloying elements in the steel. For example, while manganese is a (very) weak carbide former, it is a relatively potent carbide stabilizer. Remember, all carbide formers are also nitride formers.

Some alloying elements (e.g., Ni, Co, Al) cause graphitization of cementite (iron carbide) and for this reason are not typically added to high-carbon steels in any appreciable amount unless to counteract a strong carbide former.

In addition to forming carbides, alloying elements influence ferrite and pearlite interaction, martensite and bainite transformation, retained austenite and quenching (by influencing the alloy transformation diagram).

Carburized Steels[3]

In general, finely dispersed carbides are not considered to be detrimental to carburized alloy steels. Small spheroidal carbides or incidental carbides observed in many high-carbon martensite structures are considered routine. However, grain-boundary carbides, massive carbides that occur on edges and corners, network carbides and carbide necklaces are deemed detrimental to mechanical properties and should be avoided.

The number of carbides present in low-temperature-tempered carburized alloy steels is typically less than 10%. While carbides are harder than the surrounding matrix (martensite/austenite), they do not have an appreciable effect on Rockwell (macro) hardness at this percentage. Carbides are known to enhance wear resistance. It has also been reported that contact loading of certain types of gears (at very high contact pressures away from the fatigue limit) may be enhanced, but grain-boundary and network carbides are known to be detrimental to bending fatigue and should be avoided.

Steps many heat treaters and design engineers have found useful in minimizing carbide formation during carburizing include:

  • Use of fine-grained steels (to reduce the amount of carbon deposited at the grain boundaries) or elements that pin the grain boundaries to avoid excessive grain growth.
  • Limiting of the furnace-atmosphere carbon potential. For example, running at a carbon potential in excess of the limit of saturation of carbon in austenite (Acm) can create carbides during carburizing. Processes using these techniques must have ways to avoid carbide formation.
  • Use of a constant carbon potential (e.g., 0.80%) throughout the carburizing cycle (as opposed to a boost/diffuse cycle).
  • Avoidance of excessively long, slow cooling (i.e., drop temperature) steps in the carburizing process (to avoid network carbides).
  • Subcritical annealing after carburizing and slow cooling prior to reheat and quench.
  • Avoidance of geometry effects such as sharp corners or radii. Edge round where possible.
  • Limiting the reheat cycle temperature to avoid grain growth, excessive distortion and retained austenite.

Tempering[6]

Tempering of alloy steels differs from that of low-carbon steels in that the presence of retained austenite and strong carbide-forming elements results in the precipitation of finely dispersed alloy carbides (often referred to as temper carbides) at temperatures greater than about 500˚C (930˚F). These markedly contribute to the hardness of alloy steels. The hardness of martensite in alloy steels initially decreases as the tempering temperature is increased, but then carbon supersaturation is relieved by the precipitation of carbides (iron carbides at low temperature, alloy carbides at higher tempering temperatures where substitutional alloying allows diffusion to take place) and reaches a maximum between 500-700˚C (930-1300˚F).

Tool Steels[7]

Alloying to create large amounts of carbides is a major difference between low-alloy steels and tool steels. Tool-steel carbides have been discussed previously,[8] but the focus here is on the relative hardness of the various carbides. Tool steels contain carbon, anywhere from about 0.5% to over 2%. Tool steel with 0.5% carbon will harden into the 60 HRC range during heat treatment. Therefore, any excess carbon will combine with other elements to form carbide particles. These carbide particles are extremely small and constitute from less than 5% to over 20% of the total volume of the microstructure. The actual hardness of individual carbide particles depends on their chemical composition, but, in general, chromium carbides are 65-70 HRC, molybdenum and tungsten carbides are in the range of 75 HRC, and vanadium carbides are in the range of 80-85 HRC.

The amount and type of carbide present in a particular grade of steel is largely responsible for differences in wear resistance (for the same relative hardness, tool steels with greater amounts of carbides or carbides of a higher hardness will show better resistance to wear). Large volume fractions of carbides optimize hardness and wear resistance, but there is a trade-off with hot forming, heat treatment and machinability. Very high amounts of carbide particles present in a material (such as in tungsten carbide) can lead to problems in grinding and lower toughness.

Summary

Alloy carbides play a significant role in both the microstructure and resultant mechanical properties of alloy steels. For this reason the heat treater must pay particular attention to them, know when and why they are being formed, and if they will be helpful or harmful to the end-use application of the product.

 

References

  1. Steel Heat Treatment Handbook: Metallurgy and Technologies, 2nd Edition, George E. Totten (Ed.), “Chapter 4: Effects of Alloying Elements on the Heat Treatment of Steel,” A. V., Sverdlin and A. R. Ness, pp. 172-174
  2. Periodic Table of the Elements, Catalog No. WLS-18806-10, Sargent-Welch, VWR International, 2004
  3. Parrish, Geoffrey, Carburizing: Microstructure and Properties, ASM International, 1999
  4. Key-To-Metals (steel.keytometals.com)
  5. Maalekian, Mehran, “The Effects of Alloying Elements on Steels (I),” Technische Universitat Graz, 2007
  6. Sharma, Romesh C., Principles of Heat Treatment of Steels, New Age International (P) Limited, 1996
  7. “Selection of Tool & Die Steels,” Crucible Industries (www.crucibleservice.com)
  8. Herring, Daniel H., “Tool Steel Carbides,”Industrial Heating, January 2013
  9. Roberts, G. A., R. Kennedy and G. Krauss,Tool Steels, 5th Edition, ASM International, 1998