Melting For Clean, Sound Alloys

Most metals and alloys, particularly steel, are air melted, meaning no controlled atmosphere is involved. Air melting economically gives alloys those properties needed for less-demanding applications, and many specialty alloys are still air melted today. The two basic methods for making steel ingots are the blast furnace/basic oxygen furnace route (BF/BOF), and the Electric Arc Furnace (EAF). The blast furnace reduces iron ore to liquid metal, and the BOF produces steel. The EAF melts direct-reduced iron and scrap metal to steel. Both processes see the resulting steel tapped (poured) into a ladle, where deoxidation, decarburization, analysis adjustment and even degasification may occur. From these two primary processes, commercially pure ingots are produced.

But the end products from either route may show segregation, heavy inclusions, and porosity. 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, adversely affecting mechanical properties and hence suitability for many applications.  They may also be exposed during finish machining as machines cut into the metal. So, for the cleanest steel, we need to move to specialty, premium melting. To produce steel or other alloys with superior characteristics, methods have been developed using the primary ingots as consumable electrodes that will ensure sound material with superior cleanliness, free from segregation. These methods may be used on steel and on any alloys where soundness, homogeneity, and cleanliness are of utmost importance.

The goal of premium melting and subsequent processing is to prevent or remove defects without introducing new ones. Defects can be non-metallic inclusions, or porosity that might not be removed during hot working or finishing. Defects are not tolerated in such highly critical parts as jet engine turbine blades, aircraft landing gears, harsh environments, and medical applications. Parts with non-metallic inclusions or porosity, detected during ultrasonic testing, are rejected before they reach end users.  Rejected parts and chips go back to the furnace as charge for the next batch of steel.

Several secondary melting technologies have been developed over the years. Electro Slag Remelting (ESR) is a process that is based on an electric current running via an electrode through added molten slag and the original consumable electrode. The whole process takes place in a water-cooled copper mold. Due to the high electrical resistance of the slag, the slag heats up and melts. The consumable electrode is immersed in the liquid slag where the slag heat gradually melts the tip of the electrode. Liquefied steel drips from the electrode tip and is refined when passing through the liquid slag, with oxides and sulphur being held in the slag. After passing through the slag, the steel cools down and solidifies again into a remelted ingot. The copper mold allows the remelted ingot to solidify quickly and very uniformly. Segregation of alloying elements will be greatly improved, as will the uniformity of mechanical properties in finished products. 

Vacuum induction melting is another common process in secondary metallurgy. It makes possible the effective degassing of the melt and precise adjustment of alloy composition. The process is carried out in a furnace that is capable of reaching a state of vacuum. This vacuum is indispensable to produce high purity metals and alloys that react with oxygen. The process allows the formation of only a minimum of non-metallic oxide inclusions. Certain fabricated parts, such as jet engine parts, can tolerate only a very small concentration of non-metallic trace elements, such as oxides, or sulphides.

Vacuum induction melting enables an extremely precise adjustment of the alloy composition and the uniformity of the melt, as a result of precise control of the melt temperature and of the vacuum. Products from VIM are used in the electronics industry, in dental applications, and in the automotive and aerospace industries. They serve applications in the ferrous and nonferrous, and precious metal industries.

Vacuum Arc Remelting (VAR) is the continuous remelting, within a vacuum atmosphere, of a consumable electrode by means of an arc. The consumable VAR electrode may be generated by VIM or ESR.  A direct current is applied to strike an arc between the electrode and the baseplate of a copper mold contained in a water jacket. The very high heat from the electric arc melts the tip of the electrode and a new ingot collects in the mold. The melt rate is precisely regulated by the VAR control system. A high vacuum is maintained during the remelting process to remove impurities and prevent oxide formation. Cooling of the ingot is controlled to allow directional solidification. Strict control of temperature and of the solidification rate will provide the optimum structure in the VAR ingot.

Of the three major US steel producers, US Steel, Cleveland Cliffs, and Nucor, only Nucor is 100% committed to the Electric Arc Furnace process; the other two use a combination of Basic Oxygen and Electric Arc. 

Carpenter Technology (Cartech,) Haynes International, and Special Metals are three U.S. companies that produce high-quality alloys, for example high nickel, and high cobalt alloys, by special secondary melting techniques. 

Depending upon end use and operating conditions, the combinations of primary and secondary melting procedures may be tailored to produce alloys for an infinite number of conditions and end user applications, based on industry specifications such as ASTM, AMS, SAE, ASME, API, PED, AFNOR, GB, DIN, and customer-specific chemistry and mechanical properties (custom alloys).  All Metals & Forge Group forges many such ingots to produce parts for its customers, and can custom melt ingots for hard-to-find metals or materials with extremely long lead times, often seen in nickel alloys.