Let's talk Chrome-Moly - Part 1

Part 1 - Holy Moly

No, Chrome-Moly is not a web browser, nor is it a middle name after a complicated marriage.

Chrome-Moly, or SAE 4130 grade steel, is a type of steel that contains chromium and molybdenum alloying elements. This alloy has a better strength to weight ratio than 1020 DOM steel (which we'll talk about later). For this reason, it's used to create structures where weight is very critical, such as high-end bicycle frames, roll cages for race cars, and fuselages on small aircraft.

Chrome-Moly is one of the possible materials we can be using for our frame, because weight is an issue in the competition. In this series of articles, let's do a basic comparison between Chrome-Moly and the standard 1020 DOM (Drawn over Mandrel) steel. Both chrome-moly and 1020 DOM are typically used in the manufacture of roll cages, which makes them a good starting point for the frame.

General Pros/Cons of Chrome-Moly:

• Pro: Higher strength-to-weight than 1020 DOM
• Con: Expensive (around 1.5 times more expensive per unit length) source 1 source 2
• Con: Must be heat treated to achieve extra strength
• Con: Harder to weld than 1020 DOM (must be heated before welding)
• Con: Less flexibility than 1020 DOM

With all those cons, you'd think that Chrome-Moly is out of the picture, but it depends on how much you value weight. Chrome-Moly's typical application is to reduce the weight of a structure while maintaining the same strength as if you had built it out of 1020 steel.

The comparison boils down to strength-to-weight vs. cost vs. ease of construction.

How strong is chrome-moly exactly?

The tensile strength of 1020 DOM tube: 70ksi – 80ksi (480-550 MPa)
And now a normalized 4130 tube: 90ksi – 93ksi (620 - 840 MPa, this is after heat treating).
So 4130 is about 25% stronger. That's a bit disappointing. Where did all that godly strength go?
Maybe it's much lighter. Well, no, chrome-moly is the same weight as regular steel (491 lb/ft3, 7865 kg/m3).

So it's 25% stronger-to-weight for 50% more cost. Hmm...

But wait! It turns out that you can achieve up to 130ksi (895 MPa), but it requires a heat treating process of normalizing at 1200°F (650°C), slow cooling, heating the chassis to 1550°F (845°C), water quenching, and then a 1000°F (540°C) temper. With time and equipment, we can create a super-strong chassis that makes the most out of the cost, but is it worth it?

This round goes to 1020 DOM for now.  

In Part 2 we'll talk overview fabrication: the ease-of-construction part of our comparison! We'll also talk about typical shapes and other welding details.

Update 1: Added SI units because we're Canadian!