Performance requirements on alloys are becoming more stringent for end users in industries such as aerospace, medical, power generation, oil and gas, specialty automotive, and trucks. These requirements include few if any defects and better fatigue properties and fracture toughness. However, these properties are difficult to attain using standard air-melting techniques.
Dwight D. Wegman
Manager, Forged Bar & Billet Premium Melting
Carpenter Technology Corp.
Edited by Jessica Shapiro
• Premium melting processes create alloys with better properties and microstructures.
• Vacuum-induction melting, electroslag remelting, and vacuum-arc remelting are three common premium-melting processes.
• Premium melting can add to an alloy’s up-front costs but can also improve reliability and performance.
Carpenter Technology Corp.,
Air melting economically gives alloys the attributes needed for less-demanding applications, and many specialty alloys are still melted in air today. Some alloys, however, contain elements such as aluminum and titanium that readily oxidize when melted in air. These oxides then appear as inclusions in the finished alloy, compromising mechanical performance and making it unacceptable for critical applications.
Premium-melting processes, on the other hand, give metals producers more control over alloy composition and create cleaner, more uniform microstructures with better properties than air-melted alloys.
Raw materials are a major cost in making parts, especially when engineers specify highly alloyed and premium-melted metals. Considering material cost alone, however, provides only a short-term cost perspective. In the long run, the benefits of reliable and predictable parts made with premium-melted alloys can significantly outweigh the costs of the process.
Although air-melted alloys might have some of the attributes an engineer needs, others may be lacking or present at lower levels than required. (See Material improvement for a list of properties that can be improved with premium melting.)
The goal of premium melting and subsequent processing is to prevent or remove defects without introducing new ones. Defects can be solid inclusions, pores, or voids that might not be removed during hot working or finishing. Highly critical parts — such as jet-engine turbine blades, aircraft landing gear, and the fine wires in cardiac pacemakers — cannot tolerate defects. And materials with defects like these are usually rejected before they reach end customers, resulting in remakes and delivery delays.
Chemical segregation defects lead to alloy phases that have too much or too little of an alloying element. The result is alloy phases weaker than the parent matrix. This segregation could also become an initiation point for failure.
Low-defect microstructures can have better mechanical properties, including properties previously considered to be unobtainable or combinations of properties that do not usually exist in a single alloy. One example is high-strength alloys that also possess good fatigue resistance, toughness, and ductility.
Melting metal alloys is a common way to purify them. Impurities like oxides can be skimmed off molten metal while controlled solidification dictates the cooled alloy’s phase and grain structures.
As the name suggests, air melting involves heating an alloy above its melting point in a standard air atmosphere. In contrast, premium melting uses magnetic or electrical heat sources and may be done in vacuums or inert gases. There are three types of furnaces that make high-performance specialty alloys in wrought forms: vacuum-induction melting (VIM), electroslag remelting (ESR), and vacuum-arc remelting (VAR). (See Premium melting 101 for descriptions of each process.)
VIM furnaces are often the starting point in premium melting. The process lets metallurgists tightly control a metal’s chemistry for fewer defects and more consistent structural properties. Alloys containing oxidizable elements are usually melted with VIM to avoid forming detrimental inclusions.
Ferrous alloys requiring a high degree of cleanliness often begin in a VIM furnace and then move through one or more additional refining processes. Nickel-based alloys for aerospace and power-generation applications and cobalt-based alloys for medical applications are always vacuum melted to obtain high-purity microstructures.
ESR is a secondary process that further refines alloys formed into consumable electrodes by VIM or conventional air melting. ESR can be done at atmospheric pressure and in regular air.
ESR improves an alloy’s final microstructure by controlling solidification of the refined metal ingot, minimizing chemical segregation. Melting through slag gives metallurgists close compositional control with 0.1% variance or less for the most reactive elements.
A more-recent advancement is pressurized ESR (P-ESR) furnaces which melt alloys under several atmospheres of nitrogen. These furnaces produce steel with higher nitrogen content for different mechanical properties than do standard air-melting techniques.
P-ESR furnaces can also melt at atmospheric pressures, in air atmospheres, or with inert argon gas over the slag. Inert gas gives operators better control over reactive elements such as aluminum and titanium during melting.
Both types of ESR are used for nickel-based and ferrous alloys that must have clean microstructures. Fastener grades can also be processed with ESR. The technique is sometimes used as an intermediate step in producing alloys for aerospace components.
VAR is another secondary process for refining metal that was previously melted or remelted into a consumable electrode in an air, arc, VIM, or ESR furnace. Like ESR, VAR depends on controlled cooling and solidification of the remelted ingot to minimize chemical segregation.
VAR is the final melting step for many aerospace alloys, especially high-temperature alloys used for rotating parts, because it removes dissolved gas and oxide inclusions. Alloys used for fasteners are sometimes VAR melted for the same reason.
Both ESR and VAR use automated controls — computer hardware and software that manage every phase of the melting process. This eliminates external variables like human intervention and produces a consistent end product.
Putting it together
These techniques are part of producing high-performance alloys. The techniques are often combined with air melting or each other, as well as with additional forming, working, and heat-treatment steps to get the desired final properties.
Double-vacuum melting, which combines VIM and VAR, yields high-purity metal alloys. Aerospace and medical alloys, high-strength steels, and bearing steels are often double-vacuum melted.
Triple melting combines VIM, ESR, and VAR and takes advantage of the refining characteristics of each melting technique to produce a clean, homogenous microstructure. VIM-ESR-VAR melting is mostly used for alloys that will see high-speed rotation in aerospace and power generation.
After melting, solidified alloy ingots are reheated and hot worked to produce billet, bar, wire, rod, or hot band (flat coils). Any of these forms may be heat treated.
Hot working, cold working, and cold forming help improve an alloy’s mechanical and physical properties like yield strength, ductility, hardness, fatigue, corrosion resistance, coefficient of thermal expansion, electrical resistivity, and magnetic permeability. Technicians verify these properties with destructive and nondestructive testing.
Engineers selecting alloys and melting processes should share their plans and specifications with the alloy producer. Application engineers at the alloy vendor can either confirm the alloy choice or recommend a more suitable material. They can also guide engineers toward melting paths that will best achieve the desired properties. MD\
Vacuum-induction melting: In vacuum-induction melting (VIM), an electrode or ingot created by melting vacuum-grade revert, virgin alloys, or prerefined metal made using conventional air-melting techniques is remelted using induction. The metal is remelted in an airtight vessel from which a pump has evacuated the air. The furnace itself consists of a refractory vessel or “crucible” surrounded by a water-cooled copper-induction coil. Passing alternating electrical current through the coil creates a magnetic field that inductively heats and melts metal in the furnace.