Is there a quick way to tell ChroMo from Mild Steel?

 

Should be lighter...only way I know =/.

 

Mild steel is usually electrowelded so there will be a line running the length of tubing where the sides join and were welded together.
Moly is usually labled as such and you generally see "DOM" on the tubing. This means "drawn over mandrel" and as such is seemless. Not usually the case with mild steel, hence my tip above. Moly is also generally thinner than the same size tubing in mild steel, though special orders are available. (this explains why its lighter of course).

 

Got a microscope?

 

Yes a couple of ways.

If it's tubing, 4130 (CrMo) isn't actually Drawn Over Mandrel (DOM), it's cold drawn seamless, but it's difficult to tell the difference by looking at the finished tube. FWIW, DOM steel tubing is low carbon (mild) with carbon content from .15% to .26%. The grades are 1015 thru 1026. The 10 indicates only carbon as an alloying element. No offense intended to anyone.

You can spark test the metal on a grinding wheel. Here are a couple of links:

http://shopswarf.orcon.net.nz/spark.html

And anotlher:

"The first test I will discuss is the spark test. This is a test that anyone can perform at home. The idea is simple: the spark stream given off during a grinding operation can be used to approximate the grade or alloy of a steel. The equipment used should be a grinder with a no-load speed of 9000 rpm and a wheel size of around 2.5 inches. A semi-darkened location is necessary.

The easiest way to learn the test is to observe the spark streams from various known grades and compare them with this text. As you grind, you will see lines called carrier lines. At the termination of the carrier lines, you will see small bursts called sprigs. Low carbon (1008) is a very simple stream with few bright sprigs. The higher the carbon content, the more numerous the carrier lines and sprigs.

Some alloying elements change the appearance of the test. Sulfur imparts a flame shaped, orange colored swelling on each carrier line. The higher the sulfur, the more numerous the swellings. A spear-point shape that is detached from the end of the carrier line identifies phosphorus. The higher the phosphorous content the more numerous the spear points. Nickel appears as a white rectangular-shaped block of light throughout the spark stream. Chromium appears as tiny stars throughout the carrier lines, having a flowering or jacketing effect to the carbon burst. All said, the best thing to do is make a set of standards to use as a comparison."



This will give you the basics. Try it with a known Mild (AKA Low Carbon) Steel then your subject piece. Here's a quote about medium carbon spark testing. 4130 or 4140 (the 41 means Chrome & Molybdenum are the main alloying elements and the 30 or 40 indicate .30% or .40% carbon which is Medium Carbon)

"Low-carbon steels. Steels with a low-carbon range of 0.05 to 0.30 percent are called low-carbon steels. Steels in this class are tough, ductile, and easily machined, formed, and welded. Most of them do not respond to any heat treating process except case hardening. Low-carbon steel, when subjected to the spark test, will throw off long, white-colored streamers with very little or no sparklers.

Medium-carbon steels. These steels have a carbon range from 0.30 to 0.45 percent. They are strong and hard but cannot be worked or welded as easily as low-carbon steels. Because of their higher carbon content, they can be heat treated. Successful welding of these steels often requires special electrodes, but even then greater care must be taken to prevent formation of cracks around the weld area.

The spark test will show more numerous sparklers, beginning closer to the wheel, with the streamers much lighter in color. "

If you need to prove that a piece of metal is actually 4130 or 4140 in order to satisfy a sanctioning body, have a metallurgical lab analyze a piece of it. They can determine the alloying elements and the structure but it's not cheap. Figure a cost well over $100 depending on how much data you want. Yeah they use a microscope after they mount, polish and etch the sample. Just looking at the bare steel doesn't work.

Good luck!

 

Interesting?!

 

Just plunk a piece into your mass spectrometer

 

No assembly "seam"


David

 

OldSStroker: thanks for the very informative post.

Rich

 

No.

Both low carbon steel and low alloy chrome moly steels have a density of about 0.284 lb/in^3. Since both materials have an iron content exceeding 97% by weight, the difference in other components would create a density difference of a few tenths of a percent.

Chrome moly parts are often lighter than carbon steel, but its only because they use thinner wall thicknesses.

 

Ah, yes thats true. Since it is stronger, less material is required. Glad you pointed that out injuneer. Goes to show you I need to keep my yap shut about things I don't know .

 

Heres a thought...if you have 10 pounds of chromoly and 10 pounds of mild steel, which one is going to have less volume? Chromoloy right?

 

By less volume, you mean that the "Chrome-moly" is more dense.

Wrong, but you'll need a fairly accurate way to measure the difference.

Homework assignment: prove me right...or wrong!

Simplifying assumptions:

1) The "mild steel" is grade 1018, and is a cold finished, or cold drawn round bar exactly 1.0000 inches in diameter.

2) The "Chrome-Moly" is grade 4140, and is also a cold finished, or cold drawn round bar exactly 1.0000 inches in diameter.

3) Both grades contain the mean or averages percentages of alloying elements for each grade.

How much longer (or shorter) will the 1018 (Mild Steel) bar be than the 4140 bar if both weight exactly 10.000 lbs? If you don't care to calculate, just explain your reasoning.

Hint: "molecular weights"

 

I already gave you the answer to that one. Within 3 significant figures, the density of the two steels that I quoted was equal. If the density is equal, 10 pounds of each would have the identical volume.

In actuality, there is a slight difference in density of the two steels, because of the varying percentages of each component, including the addition of chrome, molybdenum and silicone to the chrome moly steel. But you are talking tiny differences.

 

I didnt understand what you were saying though but I do now. I love messin around with metal but I am by far not any metalogist er something! This is really great info guys, and Fred...thanks for clearing that up.