Question

1. Steel is the material that has been chosen for a small link in the fuel injection mechanism in a space vehicle. Which steelmaking process would probably be used to manufacture the steel for this application?

2. You are involved in the design and construction of a new-steel -making factory. Which steel-making process would you incorporate into this new plant? How would the geographic location of the plant affect the selection of the process?

Answer 1

1. Maraging steel is most commonly used in aircraft, rockets, and other space vehicles. now let discuss the steel making process of this material

Commercial maraging steels are carbon-free iron-nickel alloys with minor doping additions of cobalt, molybdenum, titanium, and aluminum. The word 'maraging steel' derives from the underlying strengthening mechanisms, namely, (a) transformation of the face-centered cubic high-temperature austenite phase of such alloys into cubic martensite combined with (b) the subsequent age hardening of this as-quenched material, hence, mar(tensile)-aging.

Air cooling the austenite to room temperature from about 820°C creates a soft iron-nickel cubic martensite, which contains molybdenum, aluminum, titanium, and cobalt in supersaturated solid solution. Tempering this relatively soft martensite at 480 to 500°C results in strong age-hardening due to the nano-precipitation of a high number density of intermetallic phases, including, nickel-molybdenum, iron-molybdenum and iron-nickel phases, typically with near B2 and DO3 structures, yet, with often non-stoichiometric sublattice occupation.

The standard commercial grades with 18wt.% nickel and additions of cobalt and molybdenum were developed as early as around 1950.

The standard commercial grades with 18wt.% nickel and additions of cobalt and molybdenum were developed as early as around 1950.

The first maraging steels introduced at the end of the 1950s and in the 1960s were of high interest owing to their good combination of toughness and ease of fabrication with very high strength.

THE MAKING PROCESS OF THESE STEEL

Maraging Steel is two times harder than stainless steel and 85% harder than pure titanium. Maraging steel alloys are twice as hard as stainless steel and 35% stronger than the hardest titanium alloy. On the Rockwell Scale of Hardness, stainless steel is 23-26, titanium alloys 28-41, and Maraging Steel 52-55.

Many Ni and Mn-based maraging steels with property spectra including particularly high toughness and strength have been developed.

For optimum strength and toughness, critical elements such as carbon, silicon, manganese, sulfur, and phosphorus are restricted to very low levels which renders synthesis and metallurgical refinement of such alloys challenges. Also, the number density of non-metallic inclusions must be reduced to an absolute minimum.

The maximum amounts typically specified in commercial maraging steels are (in wt. %): carbon 0.03, silicon 0.1, manganese 0.1, sulfur 0.01, phosphorus 0.01. Since the same metallurgical principles apply to all the grades the steels can be considered as a family of low carbon iron-nickel alloys containing a variety of age hardening elements.

The density of maraging steel is equivalent to 8 gm/cm3 and the melting point is 1413°C. Conventional maraging alloys are steels containing 18% Ni. These alloys are produced by vacuum melting and it can be converted into the mill product forms such as billets, bars, rods, coils, and wires. It is most suitable for die castings. It has superior ductility, toughness, low coefficient of thermal expansion and elasticity, excellent tempering resistance, good corrosion resistance, and thermal conductivity. The benefits of maraging steels are its excellent resistance to heat checking and small stresses in thermal cycling, less softening in use, less erosion, and oxidation.

2. THE STEELMAKING PROCESS:-

Integrated steelmaking operations fall into three main phases:

Reduction: ironmaking

Iron ore, as mined, is a combination of iron with oxygen and various other unwanted substances, generally known as "gangue". The first metallurgical step is to reduce iron ore to metallic iron, a process which is mostly carried out in a blast furnace, using coke as both a fuel and reducing agent. The metallic iron produced by such a furnace contains a relatively high proportion of carbon (4%) and is passed to the steelmaking process as a liquid at approximately 1450C, called "hot metal".

Refining: Steelmaking

The refining of iron to make steel is where the carbon content of the hot metal is lowered, usually to less than 1 % by an oxidation process in a steelmaking furnace. At the same time, alloying materials are added to the furnace to achieve the required chemical composition of the final product. The chemical content is controlled very closely during this stage. Originally most steels were produced by the "Bessemer" and "open hearth" processes but these have been replaced by the more modern "basic oxygen steelmaking" (BOS) and "electric arc furnace" (EAF) processes. The BOS process uses pure oxygen, injected by a lance, for refining the relatively impure hot metal (and scrap is used for temperature control). The electric arc furnace uses primarily electrical energy to supply heat to melt scrap steel, sponge iron, or mixtures of scrap and other iron units. Compared to the BOS process, the EAF requires less chemical reaction for refining.

Shaping & Coating

The liquid steel can then be cast or formed into a variety of solid shapes via the 'continuous casting' process. The cast steel can then be forged or rolled in successive steps to produce any one of the many required shapes. Rolling is the most common method of shaping. The modern rolling mill is a huge installation, costing millions of dollars and incorporating highly complex electronic control systems. The amount of work to which the steel is subjected, and the schedule on which this work is carried out, have significant effects on its physical characteristics - it dictates whether the steel can be subsequently bent, machined, cut, or subjected to any other engineering operation, or formed into tubes, pipes or wire. Once shaped, steel may be coated with other metals such as zinc or tin, or with organic coatings like paint or PVC.

The nature and location of steelworks depend on many factors:

• the availability of raw material;

• supply;

• transport (particularly deep-water port facilities);

• human resources and services available;

• markets; and often

• government factors.

In early times when coal was consumed in far greater proportions in steel production, the trend was to site integrated plants either near the coal source, or near low-grade, but cheap ore. However, with the improvements in bulk transport and the relatively greater cost of handling finished products, the trend is towards locating integrated plants on deepwater ports, perhaps thousands of kilometers from ore and/ or coal but close to markets and services. They may supply semi-finished products to more specialized finishing mills still closer to the market. The other major steelmaking method involves the melting and refining of steel scrap in an Electric Arc Furnace. Pig iron and refined iron ore pellets (briquetted iron) can also be used, and because this method is economic at lower volumes these operations are called mini-mills. Plants that use Electric Arc Furnaces can be sited closer to the market. Where market size permits, some degree of product specialization is practiced.

• I HOPE YOU UNDERSTAND THE SOLUTION WELL