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Turbine Engine Hot Section Manufacturing: Complex Metallurgy and Dangerous Work Environments

Turbine engine hot section manufacturing is a complex industry that involves risk of serious injury and an adherence to safety rules and best practices.

There is a common maxim that two technologies liberated the modern world: the automatic washing machine and the jet engine.  When RAF Lieutenant Frank Whittle received an English patent on the basic design for the modern jet engine in 1930 (the first flight was not until 1941), he probably could not have imagined the changes that would occur, in materials, complexity, and performance capability.

Today’s commercial jet engines have as many as 25,000 parts.  They are up to eleven feet in diameter and twelve feet long.  The engines can weigh more than 10,000 pounds and produce 100,000 pounds of thrust.  Even the engine on a fully tested and approved design may take two years to assemble.  A super-jumbo jet can carry 500-800 passengers, depending on configuration, and have a take-off weight of 1.2 million pounds.

Section I will provide a basic overview of the production and metallurgical complexities associated with the manufacture of some hot section components.  Section II will address a unique aspect of jet hot section manufacturing.  Specifically, the complex and exacting standards required to avoid catastrophic in-flight aviation accidents also require the most disciplined adherence to best practices for safety to avoid catastrophic occupational injury, particularly burns, in high temperature work environments.  Section III will briefly discuss the catastrophic burn injuries that result from failure to follow exacting safety precautions.

Section I:  The Hot Section

At the front of the engine, a fan drives air into the engine’s first compartment, the compressor, a space approximately 20 times smaller than the first stage of the compressor.   As the air leaves the high-pressure compressor and enters the combustor, it mixes with fuel and is burned.  As the gas is combusted and expands, some gas passes through the exhaust and some is rerouted to the engine’s turbine (a set of fans that rotate compressor blades).  The turbine extracts energy from the ultra-hot gases to power the compressor shaft and generate power.

Because the turbine is subject to such incredible heat, labyrinthine airways in the turbine blades allow cool air to pass through them to cool the turbine.  With the cooling mechanism of the airstream, the turbine can function in gas streams where the temperature is higher than the melting point of the alloy from which the turbine is made.

Titanium, purified to aviation specifications in the 1950s, is used for the most critical components of the “hot section” such as the combustion chamber and turbine.  The hardness of titanium is difficult to work with, but it is resistant to extreme heat.  It is often alloyed with other metals such as nickel and aluminum for high strength/weight ratios.

Hot Section Component Manufacturing

The intake fan.  The fan must be strong so it does not fracture if large birds or debris are sucked in.  It is made of a titanium alloy.  Each fan blade consists of two skins produced by shaping molten titanium in a hot press.  Each blade skin is welded to a mate, with a hollow cavity in the center being filled with titanium honeycomb.

The compressor disc. This is a solid core, resembling a notched wheel, to which the compressor blades are attached.  It must be free of even minute imperfections, since these could cause creeping or develop into fractures under the tremendous stress of engine operation.  Historically machined, compressor discs are now manufactured through a process called powder metallurgy, which consists of pouring molten metal onto a rapidly rotating turntable that breaks the molten metal into millions of microscopic droplets that are flung back up almost immediately, due to the table’s spinning.  As they leave the turntable, the droplets’ temperature plummets by 2120 degrees Fahrenheit (1000 degrees Celsius) in half a second, causing them to solidify and form a very fine metal powder, which solidifies too quickly to absorb impurities.  The powder is packed into a forming case and vibrated in a vacuum to remove air.  The case is then sealed and heated, under 25,000 pounds of pressure per square, inch into a disc.

Compressor blades.  These blades are still formed by traditional methods of casting.  Alloy is poured into a ceramic mold, heated in a furnace, and cooled.  The mold is broken and blades are machined to final shape, often to exacting tolerances on the order of 7 microns.

Combustion chambers.  Combustion chambers blend air and fuel in small spaces for long periods of time at incredible temperatures.  Titanium is alloyed (to increase ductility) and then heated to liquid before being poured into several complex segment molds.  The segments are welded together after cooling and removal.

The turbine disc and blades.  The turbine disc is formed by the same powder metallurgy used to create the compressor disc.  However, turbine blades are subjected to even greater stress due to the intense heat of the combustor.  Copies of the blades are formed by pouring wax into metal molds.  Once set, the wax shape is removed and immersed in a ceramic slurry bath, forming a ceramic coating.  Each cluster of shapes is heated to harden the ceramic and melt the wax.  Molten metal is then poured into the hollow left by the melted wax.

The metal grains of the blades are then aligned parallel to the blade by directional solidifying, which is important due to the blade stresses.  If the grains are aligned correctly, the blade is much less likely to fracture.  The solidifying process takes place in computer-controlled ovens to precise specifications.  Parallel lines of tiny holes are formed to supplement internal cooling passageways, either by a small laser beam or by spark erosion, where sparks are carefully allowed to eat holes in the blade.

Turbine blades are subject to temperatures of around 2,500 degrees Fahrenheit (1,370 Celsius.  At such temperatures, creep, corrosion, and fatigue failures are all possible.  Thermal barrier coatings, such as aluminide coatings developed during the 1970s, facilitated cooling.  Ceramic coatings developed during the 1980s improved blade capability by about 200 degrees F. and nearly doubled blade life.

Modern turbine blades often use nickel-based superalloys that incorporate chromium, cobalt, and rhenium.  Some superalloys incorporate crystal technology.  Nimonic is another super low-creep superalloy used in turbine blades.  Titanium aluminide, a chemical compound with excellent mechanical properties at elevated temperatures, may replace Ni based superalloys in turbine blades.  GE uses titanium aluminide on low pressure turbine blades on the GEnx engine powering Boeing 787s.  The blades are cast by Precision Castparts Corp.

Exhaust system.  The inner duct and afterburners are molded from titanium, while the outer duct and nacelle are formed from Kevlar, with all components welded into a subassembly.

Section II.  Defects in Both Hot Section Components and Safety Procedures Can Result in Catastrophic Injuries

An imperfection in the hot section, which results, for example, in a blade fracture during flight, or excessive creep, may result in an uncontrolled engine failure, among other catastrophic inflight mishaps, putting lives at risk.  In an interesting corollary, unique to very few manufacturing settings, adherence to the safest manufacturing processes will minimize both product defects and worker injuries, primarily serious burns.

Few Things Drive Higher Verdicts, Workers Compensation Costs, or Settlements, Than Burns

In those industries where “serious large burns” can arbitrarily be defined as full-thickness burns over 20% or more of the total body surface area (TBSA), the location of the burns and the relative availability of certain types of grafts can be outcome determinative and correlate directly with litigation risk, settlements, and verdicts. Most problematic are 4th degree burns to the hands or face, which can never, ever, be fully repaired with current surgical technology or therapeutic treatments.

Skin Graft Classification

There are two common types of skin grafts.  A split-thickness graft (STSG), or mesh graft, includes the epidermis and part of the dermis.  A mesher makes apertures in the graft, allowing it to expand approximately 9 times its original size.

Alternatively, a full thickness skin graft, or sheet graft, which involves pitching and cutting away skin from the donor section, is more risky in terms of rejection.  Yet counter-intuitively, this method leaves a scar only on the donor section, heals more quickly, and is less painful than split-thickness grafting.  This type of grafting, sheet grafting, must be used for hands and faces/heads where graft contraction must be minimized, and it is therefore extremely difficult to achieve in large TBSA burns.


Although workers compensation laws will generally bar litigation by workers against their employers, in cases where the exclusive remedy provision of workers compensation does not apply, it is not uncommon in the United States to see burn verdicts or settlements in the millions or even tens of millions of dollars.  Mandatory PPE and best safety practices for dealing with ultra-high temperature work environments can minimize injuries, although the practical reality is that elimination of such injuries remains an aspirational goal.

Steelmaking In The 21st Century: An Ancient Art, A Complex Modern Science, A Danger At Every Stage

Metal briefcase

Product liability lawyers should be familiar with both the dangers and the science of steel manufacturing.  Steel is one of the most indispensable products in the modern world.  Its uses, forms, and composition are limitless.  Like any other product, steel in its final form and use is a “product” subject to the same consumer expectation test in Oregon that applies to household appliances.  However, unlike most other product manufacturing, steel production, which creates the base material for pipe, rails, aviation, and innumerable transportation, mining, oil and gas, and other products, is incredibly dangerous.  Although serious burns might be the most obvious risk, there are also crush, amputation, and a host of other potential injuries which justify the most careful training, exacting safety processes, and best PPE.  This is especially true given the danger posed by the typical 24-hour-a-day production schedules and the undisputed fact that nighttime workers are in more danger than day workers.

Steelmaking Is An Ancient Art

In the ancient world, steelmaking was considered an art, and as the centuries passed, the process became more and more complex.  Steel was known in antiquity and may have been produced by managing bloomeries, or iron-smelting facilities, in which the bloom contained carbon.  Blooms are steel formed into large blocks to which further tempering or chemical procedures can be applied.  The use of blooms persists into the steelmaking of today.

The earliest known example of steel production, thought to be about 4000 years old, is a piece of ironware excavated from an archaeological site in Anatolia (the Asian part of Turkey).  “Ironware piece unearthed from Turkey found to be oldest steel.”  The Hindu (Chennai, India).  The Haya people of East Africa invented a type of furnace that they used to make carbon steel at 3,276 degrees Fahrenheit nearly 2000 years ago.  Africa’s Ancient Steelmakers (http://www.time.com/time/magazine/article/0,9171,912179,00.html?promoid=googlep).  Time Magazine September 25, 1978.

What Is Steel?

Steel is an alloy of iron and carbon.  Steelmaking is the process of producing steel from iron and ferrous scrap.  In steelmaking today, impurities such as silicon, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium, and vanadium are added to produce different grades of steel.  Limiting dissolved gasses such as nitrogen and oxygen, and entrained impurities or “inclusions,” in the steel is also important to ensure the quality of the products cast from the liquid steel.  B. Deo and R. Boom, Fundamentals of Steelmaking Metallurgy, Prentice and Hall (1993).

Carbon is the primary alloying element, and its content in steel is between 0.002% and 2.1% by weight.  Additional elements are also present in steel, including manganese, phosphorous, sulfur, silicon, and traces of oxygen, nitrogen, and aluminum.  Carbon and other elements act as hardening agents, preventing dislocations in the iron atom crystal lattice from sliding past one another.

Varying the amount of alloying elements and the form of their presence in the steel (solute elements precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting steel.  Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron.  Ashby, Michael F. and Jones, David R.H.  Engineering Materials 2 (with corrections ed.) Oxford:  Pergamom Press.  ISBN 0-08-032532-7 (1992 [1986]).

Alloys with a higher than 2.1% carbon content are categorized as cast iron.  Because cast iron is not malleable even when hot, it can be worked only by casting, where it has a lower melting point.  Steel is also distinguishable from wrought iron, which may contain a small amount of carbon.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form a number of different structures with very different properties.  One of the most important polymorphic forms of steel is martensite, a metastable phase that is significantly stronger than other steel phases.  When the steel is in an austenitic phase and then quenched rapidly, it forms into martensite, as the atoms “freeze” in place when the cell structure changes from FCC to BCC.  Depending on the carbon content, the martensitic phase takes different forms.  Below approximately 2% carbon, it takes a ferrite BCC crystal form, but at a higher carbon content, it takes a body-centered tetragonal (BCT) structure.  There is no thermal activation energy for the transformation from austenite to martensite.  Moreover, there is no compositional change to the atoms, which generally retain their same neighbors.  Smith, William F., Hashemi, Jared, Foundations of Materials Science and Engineering (4th ed 2006) McGraw Hill ISBN 0-07-295358-6.

Special Modern High Performance Alloys

There are a number of extremely complex super-alloys and other metals available today for high performance aviation and other uses, including Transformation Induced Plasticity (TRIP) steel and Twinning Induced Plasticity (TWIP) steel.  A complete discussion of these super-alloys merits a separate article, and one will be forthcoming shortly.

Introduction To The Modern Process

In the modern era, there are two major processes for making steel.  The first is basic oxygen steelmaking, which uses liquid pig iron from the blast furnace and scrap steel for the main feed materials.  Alternatively, iron ore is reduced or smelted with coke and limestone in the blast furnace, producing molten iron that is either cast into pic iron or carried to the next stage as molten iron.  In the second stage, impurities such as sulfur, phosphorus, and excess carbon are removed, and the alloying elements such as manganese, nickel, chromium, and vanadium are added to produce the steel required.  The vast majority of steel in the world is produced using the basic oxygen furnace.  In 2011, approximately 70% of the world’s steel was produced in this way.  R. Fruehan, The Making, Shaping and Treating of Steel (11th ed. AIST 1999).

The second major modern process is electric arc furnace (EAF) steelmaking, which either uses scrap steel or direct reduced iron (DRI) as the main feed material.  Oxygen steelmaking is fuelled predominantly by the exothermic nature of the reactions inside the vessel, whereas in EAF steelmaking, electrical energy is used to melt the solid scrap and/or DRI materials.

In recent times, EAF steelmaking technology has moved closer to Oxygen steelmaking as more chemical energy is introduced into the process.  E.T. Turkdogan, Fundamentals of Steelmaking, IOM (1996).  EAF steelmaking is predominantly used for producing steel from scrap and involves melting scrap, and combining it with iron ore.

Alternatively, the oxygen method can involve melting DRI using electric arcs (either AC or DC).  It is common to start the melt with a “hot heel” (molten steel from a previous heat) and use gas burners to assist with the meltdown of the pile of scrap.  EAF furnaces typically have capacities of around 100 tons every 40 to 50 minutes.

Regardless of the process used, through casting, hot rolling and cold rolling, the steel mill then turns the molten steel into blooms, ingots, slabs, and sheet.

At the typical steel mill, the raw materials are batched into a blast furnace where the iron compounds in the ore give up excess oxygen and become liquid iron.  At intervals of a few hours, the accumulated liquid iron is tapped from the blast furnace and either cast into pig iron or directed to other vessels for further steelmaking operations.  During the casting process, various methods are used, such as the addition of aluminum so that impurities in the steel float to the surface where they can be cut off the finished bloom.


The steelmaking process involves exposure to hundreds of tons of molten metals, often poured manually into ceramic, wax, or other casting forms or hot rolled into shapes.  The potential for catastrophic injury or death is everywhere in the steelmaking process, and it is essential that workers be trained and supervised to avoid lapses in safety that could result in such unfortunate occurrences.  Although automation continuously decreases the exposure of workers to significant injury or death as a result of virtually every phase of the process, the utmost care should still be exercised by all who enter a steel mill.