The Critical Role of Quality Heat Treating in Military and Defense

There is no place for mistakes in military hardware. Hardware characterised by propulsion and the items used to deliver ordnance, such as missile cases, rocket nozzles, and ammunition, need to consistently perform flawlessly. This absolute reliability is forged in fire through the science of quality heat treating. 

By fundamentally altering the microstructure of steel and other high-performance alloys, heat treatment imparts the exceptional strength, durability, and wear resistance required to withstand the violent forces of launch and detonation.

This blog looks at the specific heat treating processes necessary for mission-critical components like missile closures and aft rings, and even into the heart of advanced ammunition.

The Challenge: Withstanding Controlled Impact

Military ordnance components are designed to operate in an environment of controlled, instantaneous impact. The materials used must be able to withstand these conditions and perform with absolute predictability, as failure would lead to mission failure. This can be further broken down using three physical extremes.

Immense Pressure

Shortly after ignition, missile casings and cartridges are subjected to enormous pressures — from 1,000-5,000 psi in solid rocket motors and above 55,000 psi in rifle cartridges. Only heat treated metals are able to have the necessary tensile strength to act as ideal (yet quasi-instantaneous) pressure vessels.

Extreme Temperatures

In the process of detonation, extremely high temperatures are produced, with temperatures as high as 6,000°F (over 3,300°C) from gases produced in rocket nozzles, surpassing the melting point of most materials. Thrust and control are at risk as material strength deteriorates rapidly at these temperatures. Parts of this thermal load have to be absorbed by, for example, nozzles and blast tubes while they remain intact and keep their geometry.

Shock and Impact

Guided missiles and artillery shells experience extreme forces: more than 15,000 g during launch and jolting pyrotechnic shock from events like stage separation. These damages cause damage to brittle materials and the destruction of systems.

The Solution: Tailoring Metallurgy for the Mission

Given the extreme forces at play, raw materials are simply not up to the task. The solution lies in using specific heat treatment processes to obtain the precise metallurgical properties required for each component's unique role. 

The thermal processes for heat treating a missile casing are fundamentally different from those for a rocket nozzle or ammunition. Using detailed control over heating and cooling and restoration processes, we can play with the metal’s microscopic setup to make the final product perfect for the task.

Missile Casings, Closures, and Ends (Forward/Aft Rings)

The key positive attributes desired for missile constructional components are high strength and good toughness combined. They must be tough enough to withstand thousands of pounds of pressure per square inch — or psi — without breaking up, yet ductile enough to not shatter upon launch. The main method to achieve such a balance is Hardening & Tempering. 

First, the part is heated to a high temperature and then quickly cooled, or “quenched,” to form an extremely hard but brittle crystal structure. Tempering (reheating a part to a lower temperature after quenching it) makes it softer, but tougher and more ductile. 

Propulsion Components (Rocket Nozzles & Blast Tubes)

For parts that carry the scorching exhaust of a rocket motor, thermal resistivity is the primary obstacle. Rocket nozzles and blast tubes have to keep their exact shape, and keep from burning up at temperatures that are hotter than the alloys they are made from. The heat-treating process for this type of steel starts with annealing, where the steel is heated and kept at pressure for a period of time, and is then cooled slowly to form a through-hardening piece of steel. 

This makes the material softer and allows it to be machined to the more complex, precise tolerances that are required for the throat and bell of a nozzle. Special surface treatments like nitriding are then usually applied after the basic treatment. This treatment diffuses nitrogen into the surface of the steel, resulting in a tough, wear-resistant outer layer that is able to resist the erosive effects of high velocity exhaust gases that are spun off from the turbine wheel.

Advanced Ammunition

Ammunition cartridges face a two-fold challenge: The outer surface of a cartridge must be hard enough that it feeds reliably into the chamber and extracts easily after firing, but the overall case must be strong enough to contain the incredible pressure of detonation without splitting. The optimum response to this is case hardening. 

In this process, steel is heated in a carbon-rich environment so that carbon atoms are driven into the surface. This forms a very hard "case" or outside of the part, but leaves the parts internal core softer and tougher. As a result of this combination, the ammunition may be used with reliable operation and reduced possibility of mishap, the hard exterior resisting wear and the toughness of the interior absorbing fully the shock of firing.

Uncompromising Standards to Ensure Quality

For military parts, perfection is critical. A single flaw in heat treatment can cause catastrophic failure. This demands absolute precision, with meticulous control over every aspect of the thermal process, from furnace temperature to quenching. 

Defense and Aerospace industries depend on these essential standards to be enforced and are supported by rigorous third-party accreditations. Awards such as Nadcap (National Aerospace and Defense Contractors Accreditation Program) and compliance to Mil-Spec (Military Specifications) are more than a plaque on the wall. They are a testament to our dedication to control of process, traceability, and product quality. 

For these certifications, a heat treater’s systems, processes, and people must pass a high-level audit that verifies performance and quality control. When it comes to defense contracting, working with a Nadcap-certified heat treater is frequently not a debatable certainty, as it provides the most assurance that all parts are forged to exact perfection.

Reliability Starts with Heat Treatment

In the end, heat treatment isn't just a simple finishing step in military ordnance production; it is an entirely basic engineering process.

As we have seen, the immense strength needed in a missile shell is gained through an additional process after hardening and tempering; the extraordinary thermal resistance of a rocket nozzle starts with annealing. Each thermal process is a specially designed deliberate modification of a metal's basic structure, tailored for a particular purpose.

Since 1938, Rex Heat Treat has been dedicated to ensuring our processes yield the best results for our military and defense customers. Contact us today if you wish to explore our heat treatment capabilities further.