Introduction
Heat treatment of castings is primarily used to alter the physical, and sometimes chemical, properties of the cast alloy. Many of the mechanical properties of both ferrous and non-ferrous alloys are determined by their microstructure. Heat treatment is an efficient way of manipulating the crystalline structure of the alloy to modify its mechanical behaviour in a predictable way. Controlled heating and cooling can have an impact on properties such as hardness, strength, toughness, ductility and elasticity although changing the properties of one to achieve a desired outcome will often change another – not always in a desirable direction. However, one of the most persistent challenges associated with Heat Treatment, is the distortion of components during the quenching stage, which we have encountered with a recent project.
Overview of Heat Treatment
Quenching is the rapid cooling of a metal component during the heat treatment process, typically after it has been heated to a specified temperature, to achieve the desired mechanical properties. In ferrous alloys, this is most often done to produce a martensite transformation that will often produce a harder metal. The casting must be heated above the upper critical temperature and then quickly cooled. Non-ferrous alloys such as Copper, Aluminium or Nickel, however, will usually become softer than normal after quenching.
Quenching, and Its Complications
Like all Heat Treatment processes, quenching comes with its own challenges. We spoke to Mark Percox Technical Director at Alloy Heat Treatment, whose expertise was instrumental in helping us overcome distortion issues on this project.
“First of all it needs to be said that water is a very effective means for fast cooling (quenching) of solutionised aluminium alloy products, and is also a good choice with water being (a) abundant, (b) inexpensive and (c) environmentally disposable, without effluent cost etc.
However, water quenching can be a too effective quenchant, potentially causing (a) cracking, (b) high residual stress and (c) excessive distortion (warping).
Once aluminium alloy products are at the solution treatment temperature and key alloying elements are in solid solution, the products require fast cooling (quenching) to prevent alloying elements precipitating at grain boundaries during cooling to the quenchant temperature. If alloying elements precipitate at grain boundaries during cooling from the solution treatment temperature, then strength properties after age hardening will be lost.”
Distortion
During the project, we encountered an issue relating to the quenching medium used during heat treatment. Quenching medium controls how quickly and how evenly a part cools. One of the more common issues is the part cooling too rapidly, which can cause thermal shock, cracking, residual stress or, in this case, distortion.
We spoke with our Mechanical Engineer Tom Heal, who was assisting with this project and was able to detail the problems we faced.
“Depending on a variety of factors, such as the cooling medium, shape, size and thickness of the part and how quickly the part is submerged, cooling can occur unevenly. The result of this is internal stresses forming, and deformation can occur. For parts that are prone to this, particularly larger thinner parts, we include a process called setting. This is the method by which deformed parts are returned to flat. However, again, this can leave internal stresses on top of the stresses from quenching.
We have an ongoing project which includes a prime candidate part for deformation from heat treatment. It is a long, thin aluminium casting which needs to be flat and strong. As is procedure, these parts undergo setting at heat treatment and upon inspection before delivery to the customer they are perfectly flat.
However, we had reports from the machine shop that once machining began, the parts were bowing substantially. Interestingly, the machining process that caused the bowing was skimming the entire length flat, thus removing the outer layer of the casting. Residual stresses from quenching and setting are distributed through the section. When machining removes material, the internal stress balance changes, allowing the part to move or bow.
The cause of this is the quenching medium. The vast majority of heat treating we do uses hot water quenching. The parts are submerged into water which rapidly cools them, but the relatively low boiling point of water means that a layer of water vapour forms on the surface. This is an insulating layer which cases uneven cooling until it collapses, at which point cooling becomes much more aggressive. The fluctuating cooling causes thermal gradients and residual stress.”
What is the solution?
In this instance, the solution was to change the medium from water to a polymer-based substance. This enabled the part to cool more controllably, reducing the internal stresses, therefore improving stability during machining.
From speaking to Mark at Alloy Heat Treatment, we were able to discover the alternative mediums and why they can work so well.
“As an alternative to water and oil quenchants, polyalkylene glycols (PAG or polymer quenchants) were developed in the 1960’s for quenching of high strength thin sheet aluminium alloys into, for the aircraft industry.
Polymer quenchants are synthetic oils which are inversely soluble in water, meaning as the temperature of the water increases, the solubility of the oil in water decreases, where at temperatures typically above 70ºC, the oil becomes insoluble and separates from the water. This phenomena is particularly useful for use as a quenchant because as hot, solution treated products enter the polymer solution, oil from the solution precipitates onto the product’s surface, thus slowing down the heat transfer from the hot product into the cold polymer solution, where adjusting the concentration of the polymer solution permits tailoring of cooling rates to the needs of the products. Once products get to below 70ºC, the precipitated oil re-dissolves into the polymer solution. The polymer solution is then washed from the product’s surfaces using a cold mains water wash.
It was found by quenching aluminium alloy products into polymer solution, residual stress and quench distortion were minimised and only minor manual adjustments were needed to the freshly quenched products to make products geometrically favourable.
The downside to using polymer quenchants is that they are a relatively expensive chemicals, which have to be purchased from specific sources, which requires H+S controls and an environmental programme for compliant disposal.
At Alloy Heat Treatment, we have had many years experience of using polymer quench solutions, and have various concentration options to suit all product types and forms.”
Last Thoughts
Ultimately, the successful reduction of distortion demonstrates how carefully controlled quenching methods can improve both product quality and process reliability. At NovaCast, we are happy to discuss your project at an early stage in the design process to ensure that whatever the alloy selected or post-casting treatment applied, the end-result will meet your mechanical and performance specifications.
To discuss your requirements, call a member of NovaCast’s team on +44 (0) 1225 707466, send us a message here or email sales@novacast.co.uk or to connect on LinkedIn you can find us at – NovaCast Limited
A special thank you to Tom NovaCast’s Mechanical Engineer and Mark Alloy Heat Treatment’s Technical Director for their contributions to this article.
