Find the right metal for task at hand by comparing material characteristics: cost, workability, hardness and tensile strength. A visual guide divides 24 common types into categories and charts their relative advantages and dissapears.
There’s no such thing as “steel.” There are different type of steel, dozens of different materials ranging from carbon alloys to ones with nickel and chromium. Each type decides what your part will do, such as whether it will rust. Will it bend? Will it keep an edge?
How to Choose the Right Metal
Ferrous metals hardens primarily due to their carbon content. For example, mild structural steel like A36 contain only twenty-six percent carbon by weight. That small amount of carbon makes it easy to weld, but also leaves the metal soft enough for use in framing and bridge construction.
Steels with higher level of carbon will be harder and stronger, yet more difficult to weld. For instance, medium carbon steels (such as 1045) is used in applications requiring higher strength (e.g., axles, gears). However, preheating are required before welding to prevent cracking. This cracking can lead to a expensive failure at the jobsite.
The metal’s properties is modified by alloying metals. High-strength alloys such as 4340 and 4140 incorporate chromium and molybdenum. These alloy can be heat-treated to provide high yield strength without losing ductility (getting brittle). These grades also holds up well under shock loads, so they are used in heavy duty spindles and landing gear.
Stainless steels gets their corrosion resistance from formation of a passive oxide film rich in chromium. The table below shows the contrast between 304 and 316 stainless. Both is non-magnetic and austenitic. However, 316 contain molybdenum for better pitting resistance. So, 316 is the appropriate choice for applications involving chemical processing or other marine exposure, whereas 304 is sufficient for most indoor food-handling application. Putting the wrong type into your boat will become obvious soon, usually in just a few months… And it won’t be free.
On the non-ferrous side of things there are various benefits. For example, aluminum alloys (think 6061 and 7075) has great strength-to-weight ratio properties. The 7075 actualy has almost the same tensile strength as certain steels while being much lighter weight. It’s standard fare for aircraft applications. But aluminum isn’t as hard than tool steels.
For cutting tools or dies, try something like M2 or D2. These steels is full of carbon and other hardening elements and achieve Rockwell C hardness numbers greater than sixty. Great at holding an edge but prone to cracking if not treated carefuly and tough to machine. Pre-heating are required here.
The trick is knowing what to avoid. First, don’t lump all mild steel together, 1018 and A36 are different beasts when it comes to fabrication and stress. Don’t skimp on heat treating alloy steels. If you’re not going to harden and temper your 4340, you’ll never see its full strength and it will be a mess in anything that call for high stress. However, if you’re working with tool steels, be careful not to over-temper; you’ll destroy their hardness. It’s a fine line between good and bad.
Be sure to choose your weld filler metal to match your base material. Adhere to the carbon content limitations of welding. Apply corrosion resistance to match actual environmental conditions.
The choice of materials is a balancing act. Do I need strength without too much weight? Do I need durability without too much cost? Do I need ease of fabrication without losing performance? Each material have its strengths and weaknesses. We can see this from the chart. What we learn is that each number are a compromise. This means you won’t have to guess which material work for your application.
Guessing gets us into trouble with failing part. And whether you’re machining an exact shaft or simply making a simple bracket, knowing what makes a grade behave the way it does will mean success or failure for your project. You should of checked all specifications first.
