As heat treaters, we must be aware of what tramp elements are and how they can affect our heat-treatment operations. While certification sheets and chemical analyses performed on incoming raw material do a fine job of identifying the major chemical constituents and may indicate conformance to a particular steel standard, it is those pesky tramp elements that cause The Doctor sleepless nights. Let’s learn more.
Today, steel is a globally sourced commodity, and scrap-based steel production in electric arc furnaces is a popular steelmaking practice. The use of scrap has and will continue to grow, which means the presence of possibly more tramp elements in the future.
In simplest terms, any element that is present in the steel’s composition but not a deliberate addition is considered a tramp element. They cannot be removed by simple metallurgical processes. Tramp elements (e.g., Cu, Ni, Sn, As, Cr, Mo, Pb and others) are highly dependent on the steel grade and enter the steel through four main sources,[1] namely:
• Iron ore or pig iron
• Impurities in ferroalloy additions
• Refractories
• Scrap steel
An important distinction must also be made between tramp elements that have an effect due to their presence in solid solution (e.g., Mo, Cr, Ni, Cu) and those that have an effect due to their segregation at interfaces such as surfaces and grain boundaries (e.g., Sn, As, Sb). In addition, elements with an atom size smaller than the solvent atoms (e.g., C, N, B) may also segregate at grain boundaries, competing with tramp elements and often protecting the interfaces from possible detrimental effects due to residual enrichment.[1]
Annealing and Mill Effects[1,2]
In general, tramp elements contribute to an increase in strength with a resultant loss of ductility. Thus, casting, forming, drawing and annealing operations performed at the mill can be affected (Table 1). For example, molybdenum and chromium present in extra-low-carbon steels increase resistance to hot deformation, requiring higher rolling loads. The presence of tin and arsenic will adversely affect recrystallization kinetics during continuous annealing of certain cold-rolled steel grades and require an increase in annealing temperature.
Copper, one of the most recognizable tramp elements, is responsible for surface defects related to scaling and cracking (hot shortness). It is a good indicator of the amount of scrap used in the steelmaking process, often present at levels of 0.20% or greater. Nickel, if present in about the same percentage, offsets the Cu effect, but Sn and As amplify it. The presence of even minute amounts of tin (0.05% added to a steel containing 0.22% copper) increases the tendency toward cracking.
Quench and Temper Effects[2]
Downstream processing and final properties are also affected by tramp elements (Fig. 1, Table 2). Grain-boundary embrittlement (Table 3), which can occur in even low-alloy structural steels, is one such property and is a function of heat treatment and composition, with major alloying elements and residual (tramp) elements playing an important role. Most embrittlement occurs in steels in the quenched-and-tempered condition with a martensitic structure. This susceptibility is reduced somewhat in bainitic, pearlitic or ferritic structures.
Grain-boundary embrittlement can manifest itself as temper embrittlement, stress-corrosion cracking, hydrogen embrittlement, creep rupture, stress-relief cracking and fatigue-crack growth. Very small total amounts (<200 ppm) of residuals can cause embrittlement, but concentrations are often a function of heat-treatment time and temperature. The types of embrittlement elements responsible (Table 3 – online only) can be categorized as:
• Class 1: Elements that are the primary cause of embrittlement
• Class 2: Elements that enhance embrittlement
• Class 3: Elements that improve grain-boundary cohesion
• Class 4: Elements that inhibit grain-boundary embrittlement
For example, Class-2 elements are necessary for temper embrittlement to occur. Recall that temper embrittlement (TE) is a result of cooling slowly through the temperature range of 350-575°C (660-1070°F) or holding in the range for relatively long times. Certain elements (e.g., Mo, La, Ce) have been found to reduce temper-embrittlement effects but may affect other properties, such as fatigue (especially the rare-earth elements).
Finally, segregation (micro or macro) can increase the level of both alloying elements and trace elements (especially P, S, Sn and As) and has been found to adversely affect certain properties. Toughness and the heat-affected zone in weldments are typical examples. Casting practices are extremely important to help negate segregation effects.
Carburizing Effects[4]
At elevated temperatures, some of the impurities present in low-alloy steels tend to segregate in areas such as grain boundaries and at the near surface. The case depths from gas carburizing, for example, can be negatively impacted since the carbon transfer is impeded.
In one experiment,[4] a 16MnCr5 steel (SAE 5115) was gas carburized at 930°C (1700°F) in an atmosphere of CO-H2O-H2-He. A carbon potential of 1.3% was achieved after 1,000 minutes for the standard alloy (0.0025% Sb). Increasing the antimony content to 0.017% (170 ppm) extended this time to 2,000 minutes and increasing it to 0.058% (580 ppm) extended the time to 6,000 minutes.
The following sequence was found for the retarding effect on gas carburizing of the these tramp elements: Sb > Sn > P > Cu > (Pb).
In some instances, such as high-temperature carburizing at 1040°C (1900°F), it is advantageous to deliberately add certain trace elements (e.g., Al, Nb, Ti, N) to pin the grain boundaries and help prevent excessive grain growth.
Final Thoughts
Tramp elements often come to our attention when solving heat-treatment problems, where it is critical to know the full chemistry of the material under investigation (and not just the chemistry as reported on the mill certification sheets). Often, the prior heat-treatment history is also highly relevant.
Finally, it is important to understand how each tramp element affects the steel, either singularly or in combination with other elements. Going forward, by knowing the tramp elements present, we can better design our heat-treatment recipes and avoid costly failures. And The Doctor will sleep better at night. IH
References
1. Herman, J. C., and V. Leroy, Influence of Residual Elements on Steel Processing and Mechanical Properties, Metal Working and Steel Processing Conference, 1996.
2. Effects of Tramp Elements in Flat and Long Products, European Commission on Technical Steel Research, 1995.
3. Mohrbacher, Hardy, Effect of Tramp Elements in Unalloyed Low Carbon Steel, Conference Presentation, TAMOP-4.2.1/1-2008-0016, 2008.
4. Ruck, Andreas, Daniel Monceau and Hans Jürgen Grabke,Effect of Tramp Elements Cu, P, Pb, Sb and Sn on the Kinetics of Carburization of Case Hardening Steels, Steel Research 67 (1996) No. 6.
5. Krauss, George,Steel: Processing, Structure, and Performance, ASM International, 2005.
6. McLean, M. and A. Strang, Effects of Tramp Elements on Mechanical Properties of Superalloys, Materials Science and Technology, Volume 11 Issue 1 (01 January 1084), pp. 454-464.
7. Nickel, Cobalt and Their Alloys, Joseph R. Davis (Ed.), ASM Specialty Handbook, ASM International, 2000.
