Chromoly frame grade?

http://www.industrialmetalsupply.com/products/chromoly-4130-dom-tubing

a quick Google search says this:

A. 4130 is a designation for a steel alloy that contains chromium and molybdenum among other elements. The exact composition range is defined in the standard SAE J404 [link is to spec at TechStreet]"Chemical Compositions of SAE Alloy Steels".
1020 is another steel alloy, but since it does not contain appreciable amounts of alloying elements other than carbon, silicon, & manganese, it is called a carbon steel, not an alloy steel, and its composition range is defined in the standard SAE J403 [link is to spec at TechStreet]"Chemical Compositions of SAE Carbon Steels".
So, to answer your question, there is no difference between "100% Chromoly" and "4130 Chromoly" except that one description refers to an industry standard and the other one is pretty much meaningless.
Toby Padfield
Automotive module supplier - Michigan

or

A. The 4130 is a code of the American Iron & Steel Institute and defines the approximate chemical composition of the steel.
The "41" denotes a low alloy steel containing nominally 1 percent chromium and 0.2 percent molybdenum (hence the nickname "chromoly"). The "30" denotes a carbon content of 0.30 percent. In addition, as normal constituents of plain carbon and low alloy steels, there will be around 0.2-0.5 percent silicon, 0.5-1.0 percent manganese, and well under 0.1 percent of each of a dozen or so other elements whose presence is unavoidable, in a few cases deliberate, and generally not harmful. The remaining 97-98 percent is iron (Fe).
There are many tens, perhaps hundreds, of different steels defined by that AISI 4-digit code system. The code does not specify any particular mechanical properties, only approximate chemical composition. For any particular grade (composition) (AISI number) of steel, the mechanical properties depend on what heat treatment has been applied, and what (if any) cold work has subsequently been done.
Bill Reynolds
consultant metallurgist
Ballarat, Victoria, Australia

And

A. The chromium content is approximately 0.8-1.1%. The carbon content is nominally 0.30% and with this relatively low carbon content the alloy is excellent from the fusion carbon content the alloy is excellent from the fusion heat treatment. The actual breakdown of 4130 alloy steel is as follows:
Carbon 0.28 - 0.33
Chromium 0.8 - 1.1
Manganese 0.7 - 0.9
Molybdenum 0.15 - 0.25
Phosphorus 0.035 max
Silicon 0.15 - 0.35
Sulphur 0.04 max

Some other facts regarding this material:

Applications
Typical applications for 4130 low alloy steel include structural use such as aircraft engine mounts and welded tubing applications.

Machinability
This alloy is readily machined by conventional methods.
Machinability is best with the alloy in the normalized and tempered condition. Although the alloy may be machined in the fully heat treated condition, machinability becomes more difficult with increasing strength (hardness) of the alloy.

Forming
Formability is best in the annealed condition for which the ductility is very good, Bend radii of 1t on annealed sheet material may be made.

Corrosion Resistance
4130 is a steel and as such is not corrosion resistant. In corrosive environment the alloy should be given a protective coating.

Welding
4130 alloy is noted for its weldability by all of the commercial methods.

Heat Treatment
Heating at 1600 °F followed by an oil quench will harden the 4130 alloy. For best results a normalizing pre-hardening heat treatment may be used at 1650 to 1700 °F followed by the 1600 °F soak and oil quench.

Forging
Forge at 2200 °F maximum down to 1750 °F.

Hot Working
4130 in the annealed condition has excellent ductility.
Thus it is usually not necessary to do hot working to form parts. If hot working is needed it can be done in the range of 2000 °F to 1500 °F.

Cold Working
Cold working by conventional methods is readily accomplished on this alloy.

Annealing
4130 (and most of the other low alloy steels) may be annealed at 1550 °F for a time long enough to allow through heating of the section size. It should then be cooled in the furnace at a rate of less than 50 °F per hour down to 900 °F, followed by air cooling from 900 °F.

Tempering
Tempering is done to restore some of the ductility that may be lost after the hardening heat treatment and quench. Alloy 4130 is tempered at between 750 °F and 1050 °F, depending upon the strength level desired. The lower the tempering temperature the greater the strength.

Other Comments
The 4130 alloy is a through hardening alloy and should not be case hardened.

If you have any other questions or comments regarding steels of any type, please leave me a message and I will try to answer you -- a bored Tool and Die / Toolmaker that has worked with many different materials and applications over the years.
Jack Demcovich
- Webster, New York

Husky John