Heat Treatment is a very important process in the world of material science and engineering. It is the processing by which the mechanical properties of materials can be enhanced and the resulting metallic materials can be used to create new structures and machines for our world.
As, metals form the backbone of our modern world, the techniques used to change or enhance their properties are also important.
However, when it comes to heat treatment process, often people get confused between two terms, which are solution heat treatment and precipitation hardening.
Well, if you are confused about the difference between the two techniques that fall under the umbrella of heat treatment? Then this article is for you.
What is Heat Treatment?
Before we delve into solution heat treatment and precipitation hardening, understanding the process of heat treatment itself is essential.
Heat Treatment is a group of metalworking techniques used to alter the mechanical properties of metals.
How it is done? By cycles of heating and cooling the metal.
The basis of heat treatment technique lies on the fact that by changing the temperature and ratio of impurities in metallic alloys, its microstructure can be changed. And different microstructures have different mechanical properties, hence those properties can be altered via heat treatment.
Let’s take example of steel. Steel is the backbone of modern world as it is the basic component for all of our structures and machines.
Steel contains carbon as the impurity element.[Read More about Classification of Engineering Materials here]
The road-map the tells us how the temperature of a steel is to be changed to get the micro-structure/properties that we want is called the Iron-Carbon Phase Diagram.
Ferrite, Austenite, Cementite and there are also other like Martensite and Pearlite, are microstructures of steel.
Now, For example. if you have steel with 0.8 % carbon content and heat it up to lets say 600 degree Celsius, then cool it town through the transition states to room temperature, you will get a steel with cementite and ferrite mixture, called a pearlite.
So, basically, phase diagrams give you a roadmap for achieving the desired microstructure.
Cooling can usually done by following three methods:
- Annealing:Cooling down inside the unfired furnace. Slow Cooling.
- Normalizing: Cooling down in room temparature. Medium Paced Cooling.
- Quenching: Rapid cooling in water or oil.
Solution Heat Treatment
Solution heat treatment is called as solution annealing. It is one of common heat treatment technique. It used for many family of metals and their alloys. Most common of them are steels, nickel based alloys and titanium based alloys.
The objective of solution heat treatment or solution annealing is to dissolve the precipitates or impurities present in the material.
The key point to remember is that in solution heat treatment, alloying effect takes place while remaining inside one phase. Meaning during solution annealing process, as the name suggests, employs annealing mentioned above, and while slowly cooling without changing the phase, precipitates are made to dissolve.
After the precipitates have been dissolved, the metal alloy is rapidly cooled down via quenching, to avoid the occurrence of precipitation during the cooling down stage.
The solution annealed or solution heat treated material goes into soft state after the treatment process.
Precipitation Hardening (AKA Age Hardening)
Precipitation hardening, also called as age hardening or sometimes particle hardening, is one the most commonly used strengthening mechanisms for metals.
Strengthening by precipitation hardening occurs as the process produces uniformly dispersed particles (precipitates) within grain structure of metal. Thus, these particles limit and hinder the movement of atoms within grain structures, consequently limiting the dislocations occurring in the metals. Therefore, by this metals becomes hard and brittle.
Precipitation hardening is done by taking the resulting material from Solution Heat Treatment, which is after being in soft-state, as mentioned above, and then heat it up again to medium temperatures and then rapidly cool it down again via quenching. Thus, you get a hard and strong material. Now, precipitation hardening is to be done.
This done by placing the material that has been treated to solution heat treatment, in vacuum, inert atmospheric temperatures of 480 to 620 degree Celsius.
Time duration for the process can vary from 1 hour to many hours depending the characteristics of material.
This process produces small precipitates and disperses it uniformly withing the grain structure of material.
The conditions for a material to undergo precipitation hardening are following.
- Appreciable maximum solubility
- Solubility must decrease with fall of temperature
- Alloy composition must be less than the maximum solubility.
So, what is the difference between solution heat treatment and precipitation hardening? Well, you can say that solution heat treatment is a step in performing precipitation hardening.
And, the product of precipitation hardening is hard and strengthened material, while other produces comparatively ductile and soft material.