Alloying elements in cast steel

Before any alloying element comes carbon, the element that defines the basic behavior of steel. The more carbon, the more hardness and strength the part can reach, especially with heat treatment. But there is a trade-off: the higher the content, the lower the toughness and weldability. That is why high-hardness wear steels carry more carbon, and those that must resist impact or be welded carry less. Everything else is built on that base.

Chromium is the most versatile alloying element. In high proportions (12–18% and up) it forms the passive layer that gives the corrosion resistance of stainless steels. In smaller proportions it improves hardenability and hardness, and in high-chromium steels it provides great abrasive wear resistance. It also sustains the high-temperature properties of heat-resistant grades. It is of moderate cost, but in the high percentages of a stainless it weighs on the final price.

Manganese plays two roles. In small doses it is a deoxidizer and improves hardenability, present in almost every steel. In high doses (austenitic manganese steels) it gives something unique: the part hardens as it takes blows —work hardening—, so it resists impact and severe wear without becoming brittle. It is also one of the most economical alloying elements

Nickel provides toughness: it lets the part absorb impacts without cracking, and especially at low temperatures, where other steels become brittle. In austenitic stainless steels it is the element that, together with chromium, stabilizes the structure and adds corrosion resistance. Its downside is price: nickel is expensive and its quotation is volatile, so raising its percentage hits cost directly and unpredictably.

Molybdenum solves specific problems. It improves hardenability in thick sections —it lets large parts harden evenly to the core—, increases high-temperature strength (creep resistance) and, in stainless steels, improves resistance to pitting corrosion in chloride environments. It is an expensive alloying element, used in measured doses and only when the application justifies it.

Every element added improves a property and raises the cost. Manganese and chromium are relatively affordable; nickel and molybdenum are the ones that add the most, and nickel also fluctuates in price. That is why specifying more alloy than the part needs is throwing money away, and specifying too little risks a failure. The right alloy is not the most «loaded» one: it is the most economical one that meets the part's service.

Carbon, chromium, manganese, nickel and molybdenum are not «better» or «worse»: each one is a tool for a specific property, with its associated cost. Designing an alloy is adding what the part's service needs and nothing more. CYM Materiales defines that combination according to the application and verifies the composition of every heat with a spectrometer, so the part carries exactly the elements specified.