Knowledge

Heat treatment is a crucial step in ensuring the mechanical properties of cast steel parts. Through forming, pouring, shakeout and cleaning, the casting takes its final shape - but it may not be strong or flexible enough for final use. By heating and cooling the metal at different rates, foundries can alter its mechanical properties. But how does the application of heat change the strength or flexibility of the metal? ​​​​​​​

Crystallization and Metal Properties

When molten metal cools, it solidifies into a crystalline structure. Under a microscope, these structures resemble the frost patterns that form on glass in winter. Each structure grows from a central point until it meets another crystal structure. These structures make up the "grains" of the metal.

Just as different winter conditions produce various types of frost patterns, different temperatures also alter the crystals formed when making metals. The grains they produce are usually invisible but become apparent when the metal is etched with acid.

The shape and relationship of grains in an alloy determine its mechanical properties. When metal is struck, round grains can slide past each other, denting rather than remaining firm or breaking. Flat grains can stack like bricks in a wall and support each other; they are stronger than round grains but still allow some movement. Jagged, interlocking grains may offer no give at all. Heat treatment of metals can reshape their crystallization, thereby altering their grains and thus the properties of the metal.

Work-hardened metals

The image of a blacksmith hammering a glowing metal plate in his forge, although no longer common, is instantly recognizable. However, for most of human history, blacksmiths mechanically worked metals to make them stronger. Today, steel is no longer hand-forged by blacksmiths but is often rolled to mechanically harden it.

A depiction of the grain structure explains the effect of work hardening. The round particles within the metal deform, and their new shapes give the metal strength. For example, in cold rolling, the round grains are squeezed and stretched, becoming more like rods. These rods support each other, like a bundle of sticks. A blacksmith or metalworker can hammer, twist, heat, cool, and stretch an object to change the shape of the grains. If the grains have nowhere to go when struck, they form an immovable inelastic matrix, thereby increasing the hardness of the metal.

However, this hardness can come at a cost: strength may make the material brittle. Irregularly shaped grains do not slide past each other easily: they are wedged together. Any impact large enough - greater than the strength of the bonds between the grains - will separate them.

Heat-treated metals

Foundries began to create the desired mechanical properties of steel by choosing an alloy known to produce these characteristics. However, as the castings cooled, the crystallization of this metal was almost impossible to control. Since crystallization affects the mechanical properties of the metal, the alloy may not perform optimally unless further processed. Foundries can achieve this by heating and cooling the metal in a controlled and regular manner.

Heat treatment is a non-destructive method of altering the properties of materials. It is sometimes a secondary process for work-hardened metals - but it is the preferred method for foundries because the castings are already in the correct shape and cannot be machined.

Crystallization almost always begins at the outer surface and moves inward, and - especially in large castings - there is a significant temperature difference between the shell and the center of the casting. Crystals grow irregularly, usually being sharper and less ductile near the surface. They are usually more rounded, and thus softer the further in they go. The shape of the casting and defects or inclusions in the metal can affect the cooling rate, resulting in different mechanical properties in different areas of the metal. These differences can cause internal metal strain, leading to metal fatigue or failure. Heat treatment allows foundries to go back into the metal and rearrange the crystals that make it up.

Soaking is the process of bringing a casting above the recrystallization point. The specified "temperature time" for soaking in heat treatment allows the crystals in the metal to melt and reorganize. Reviewing the iron-carbon phase cycle can help foundries understand how long to keep a casting at a certain temperature to allow for specific carbon diffusion.

In most (but not all) parts of the iron-carbon phase cycle, soaking cast or processed metals reduces their hardness and brittleness. As the particles in the metal grow more regularly, they become rounder and can rearrange upon impact by sliding past each other. Additionally, since the item always reaches the same temperature, the crystals are usually more uniform than those in newly cast pieces.

Annealing

Annealing begins with soaking and continues by very slowly cooling the steel in the furnace. The foundryman turns off the furnace and allows the temperature to drop gently and controllably. During heating and cooling, the entire object has thermal consistency, which means there is very little internal stress: there will be no metal "regions" with different crystalline properties. Annealed metals usually have very good ductility, with increased ductility, tensile strength and elongation. Due to the very slow cooling curve, the grain size of annealed metals is usually very large.

Normalizing

Normalizing metal involves heating it to the recrystallization temperature and then pulling it out of the furnace and cooling it in the atmosphere. Many of the properties of annealed metal are also present in normalized metal, but due to the incomplete uniformity of cooling, the grains are often less regular. Nevertheless, the temperature difference is much smaller than that in chilled metal, which means that standardized products are less brittle.

The cooling rate found in normalizing produces smaller grains in the metal than annealing, which means that it is usually stronger or harder than annealed metal.

Quenching

What if extremely high hardness is required? In the manufacturing of tools and machine parts, softening the metal may defeat the purpose.

Heat treatment can allow for specified and consistent hardness. To make steel hard, foundries immerse the steel until austenite becomes the dominant molecule and then quench it in cold oil or forced air. When austenite is subjected to a cold shock, it forms a slightly irregular crystal structure called martensite. Due to the deformation of carbon in each martensite molecule, this material is harder.

Since quenching occurs from the outside in, large objects are subject to the pressure of rapid crystallization, which can cause internal stress in the metal. If quenching is too extreme, these forces can sometimes cause cracking. For this reason, water quenching is not common for large steel objects because it causes a rapid drop in temperature, leading to the formation of cracks. Oil and air cool slightly less aggressively.

However, quenching hardens more than just steel. Water quenching is used in foundries. Non-steel metals may not withstand the same internal pressure because their phases and molecular structures are different. Manganese can be water quenched at a much higher temperature than steel without cracking. However, the temperature difference is so great that any quenching would handle a large amount of energy that could go wrong! Below is an explosion during the quenching process of a manganese steel casting caused by a residual sand core.

Tempering

Finding the right balance of hardness and ductility can also be achieved through a process called tempering. Tempering is usually done on quenched steel to make it less brittle while retaining some hardness. In tempering, the metal is reheated, but at a temperature lower than that used in annealing, normalizing, or quenching.

Martensite is not a stable molecular structure at high temperatures—it forms under shock—so tempering steel means making the martensite unstable and causing it to start transforming into cementite and ferrite. The temperature range and duration in the tempering furnace will affect the extent of the martensite transformation, and thus the softness of the metal. For example, metal springs can be tempered at a higher temperature to increase elasticity compared to tools that are tempered at a lower temperature to maintain hardness.

Tempering is often used to relieve internal stresses in quenched materials. A metal that has undergone other thermal stresses, such as welding or forging, can be tempered to allow the internal molecules to relax slightly.

Heat treatment variations

In foundries, castings are usually subjected to uniform heat treatment. However, sometimes items may undergo irregular heat treatment. Tempered steel swords typically undergo different degrees of tempering to make the blade hard while keeping the core elastic. Springs sometimes undergo different heat treatments to match their functions.

Vigorhas more than 18 years experience on heat treatment of castings, forgings and other metal parts.If you have any question and demand of products development or improve your supply chain, please feel free to contact us atinfo@castings-forging.com