In earlier posts, we explored how alloying elements and microstructure affect the mechanical strength and corrosion resistance of various alloys. In this post, we’ll focus on a key process that enhances these properties: precipitation hardening. Also known as age hardening, this is a thermal treatment used to increase the strength and yield strength of certain alloys. However, it’s not a one-size-fits-all solution for every material.

At Columbia Metals, we stock several alloys – like Colsibro®, beryllium copper, 17-4PH, and Alloy 718 – that benefit from precipitation hardening.

What is Precipitation Hardening?

Precipitation hardening is a three-step heat treatment process. Each step plays a vital role in developing the final properties of the alloy, and slight adjustments can fine-tune the outcome. Let’s break down the stages:

Stage 1: Solution Treatment (Solution Annealing)

In this first step, the alloy is heated to a high temperature, allowing the alloying elements to dissolve into a uniform single-phase solution. This “resets” the alloy’s microstructure, ensuring an even distribution of elements throughout the metal.

• Why is this important? A consistent microstructure ensures uniform mechanical properties across the material.
• Key considerations:
  • If the material is heated too low, not all elements will dissolve, resulting in coarse particles that cause uneven distribution of precipitates, which weakens the material.
  • If the material is heated too high (above the eutectic temperature), pockets of alloying elements can become overly concentrated and start to melt. This creates weak spots and structural defects that can harm the material’s performance after heat treatment.

Stage 2: Quenching

Immediately after the solution treatment, the alloy is rapidly cooled, or quenched, to lock the alloying elements in place. This creates a metastable, supersaturated solid solution.

• How is quenching done? Depending on the alloy, different cooling methods are used, such as water, oil, or air quenching. Each method is chosen to meet the specific cooling rate required to maintain the desired microstructure.

Stage 3: Ageing

After quenching, the material undergoes the final step – ageing – where it is reheated to a lower temperature for a controlled amount of time. During this phase, small particles called precipitates form and distribute throughout the alloy.

• Why does this improve strength? The precipitates block dislocations in the metal’s crystal structure, making it harder for the material to deform, thus increasing strength and resistance to plastic deformation.
• Fine-tuning the process:
  • The temperature and duration of ageing are carefully controlled based on the alloy. For example, a process called double ageing is used for some alloys, like Alloy 718, to achieve even better properties by refining the size and distribution of the precipitates.

Underaged materials have smaller precipitates, while overaged ones develop larger, coarser particles, which can lead to reduced strength.

Advantages of Precipitation Hardening

Precipitation hardening significantly boosts an alloy’s tensile and yield strength by preventing dislocation movement. It offers key advantages over other strengthening methods, like work hardening:

• Machining flexibility: The alloy can be machined while in its soft, solution-treated condition, and then hardened through heat treatment to reach its final strength.

Improved mechanical properties: The treatment results in a stronger, more durable material that’s better equipped to resist plastic deformation.