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Author: Subject: What's precipitation?

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[*] posted on 15-11-2017 at 07:42
What's precipitation?

Yes, I have googled that question and no I haven't found a good explanation so I was hoping that a science forum could help.

I've come across this term while researching hardening options for a client I'm working for. He's trying to find a way to improve wear resistance on water fittings and has asked me to research this technique. While I am a part-time consultant on material procurement but usually I help my clients with other things. Not exactly an expert on metals, I'm afraid but it's an opportunity to learn I suppose :)
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[*] posted on 15-11-2017 at 08:09

Here's a small crash-course on metallic precipitates:

Metals, in general, are polycrystalline in form. Many crystals, referred to as 'grains', are grown in the bulk of the material during thermal treatments by diffusion mechanisms, 'precipitating' out of the melt or the heated, larger grains of a different phase. However, despite generally being of a different composition and phase than their parent material, precipitates require atoms to form, which diffuse through the bulk metal to arrive at the proper sites to grow (usually the boundaries between the old parent grains). In the case of the website above, this reduces corrosion resistance because the precipitates in question (chromium nitride and chromium carbide) deplete the bulk material of chromium, which forms a passivating oxide layer on the bulk material's surface that resists corrosion.

However, this strictly refers to corrosion of a chemical nature - abrasion and wear resistance is determined by the hardness and mechanical properties of the material, which is usually a function of composition. In fact, precipitates are often known to strengthen alloys by a process known as 'precipitation hardening,' because they block the movement of plastic dislocations and thus prevent plastic deformation from occurring at lower stresses. Brittle elements such as manganese improve abrasion resistance by increasing hardness and elastic modulus, while softer elements do just the opposite. Therefore, selecting a material for water fittings according to wear resistance is merely a question of 'what alloy is hard but also cheap,' with additional restraints imposed as necessary. Generally, a good idea is stainless steel, as it can be both quite hard and quite corrosion-resistant. However, other alloys are generally less expensive.

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