Originally published January 1998
I used to wonder why 4130 steel tubing was ubiquitous in welded tube fuselages and other airplane parts. What with a lot of talk on the Bearhawk e-mail about welding, whether to stress relieve or not, etc, I decided to look up 4130 and determine if my memory about it was true.
I remembered that it was a chrome-moly (chromium-molybdenum alloy) steel but forgot that it was referred to as a "a low carbon alloy" steel. After researching it a bit I realized that it has what I consider very unique characteristics that make it suitable for welded tube structures.
The 30 in 4130 represents a value of 0.30 percent carbon content. It is a general cut-off value for weldability. Alloys with more than 0.30 percent carbon are not as easily weldable.
A person new to welding will likely say, "I can weld 4140 just as easy as 4130." What is meant is that above 0.30 percent carbon, it is difficult to make a weld that does not crack or does not have hidden inclusions or occlusions in the weld.
4130 is not for all aviation applications. I believe that it is the best for welded tube fuselages and applications with material thicknesses up to 1/8 inch thick. It is has a tensile strength of 90-95 ksi (ksi = 1000 pounds per square inch) in the normalized condition with a maximum heat treat (in an oven) of 180 ksi. Most parts designed by aircraft manufacturers (especially of high performance aircraft) use alloys capable of 220 to 240 ksi. Many aircraft are now using alloys at 300 ksi for certain applications..
Sometime we will talk about tradeoffs between ductility, strength, weight, and fabrication advantages for welded tube fuselages (and corrosion protection).
Most of the words below are direct from References 1 and 2.
AISI 4130 is a chromium-molybdenum steel that is in general (aviation) use due to its well established heat-treating practices and processing techniques.
AISI 4135 has a slightly higher carbon version of AISI 4130. AISI 4140 is a chromium-molybdenum steel that can be heat treated to higher strength levels or in thicker sections than AISI 4130. AISI 8630, 8735, and 8740 are nickel-chromium-molybdenum steels that are considered as alternates to AISI 4130, 4135, and 4140, respectively.
But in all the similarities of the alloys, the following discussion is only for 4130. Any changes from 4130 will cause one of its advantages to deteriorate.
4130 is used in the normalized or near normalized condition and does not require heat treatment. In the normalized state the maximum allowable tension stress is 95 ksi. At this value 4130 has good toughness and excellent elongation. Toughness means resistance to crack propagation and elongation means it can absorb energy in deformation without breaking (useful in a fuselage safety cage).
By letting weld joints air cool, the joint becomes normalized. Further normalizing and evening of the internal stresses can be accomplished by heating the general area of the weld (cluster) to a straw color (light yellow) and letting the area air cool. This is not absolutely necessary (because the internal stresses of the weld are not that much higher) like it would be with a higher carbon content steel, but I was taught to do it.
This heat-treatable low-alloy steel has relatively low hardenability; nevertheless, it is one of the most popular alloy steels because of its good formability and weldability along with an excellent mechanical properties.
Its optimum combination of properties is developed in limited section thickness’ by quench-and temper heat treatment, but sufficient strength and toughness for many applications can be obtained by normalizing.
It is recommended for use at temperatures up to 700° F because its strength decreases markedly with increasing temperatures above that level. At sub-zero temperatures it undergoes a transition from ductile to brittle behavior as shown by a Charpy V impact test where it exhibits poor impact properties. The transition temperature varies with the heat treatment.
This steel is not subject to temper embrittlement, and can be nitrided. (Nitriding gives a hard surface used in wear applications such as gears.) It is usually forged at 2000° F to 2200° F and the finishing temperature should never fall below 1800° F. For applications in moist, marine and other corrosive environments, paint, electroplate or other protective coatings should be applied to inhibit rusting and other corrosion.
It is used in both cast and wrought form for many applications requiring high strength and toughness.
4130 steel is available as billet, bar, rod, forgings, sheets, plate, tubing, and castings. It is used to make automotive connecting rods, engine mounting lugs, shafts, fittings, bushings, gears, bolts, axles, gas cylinders, airframe components, hydraulic lines, and nitrided machinery parts.
For critical applications requiring high strength and toughness, therefore, use should be made of precise hardenability calculations to determine the suitability of specific heat compositions.
4130 is susceptible to hydrogen embrittlement in chrome, nickel, and cadmium plating solutions and in hydrochloric-acid pickling. It is generally not susceptible to alkaline cleaners or anodic acid cleaners. Baking at 375° F for times up to 24 hours, depending upon the severity of the embrittlement, is effective in relieving the embrittlements.
Sheet and strip are extensively used where minimum tensile strength of 180,000 psi is required in sections up to 1/8 inch in nominal thickness.
Under the relatively new DoD directives, the Military Specifications for 4130 have been cancelled in favor of commercial standards such as Society of Automotive Engineers Aerospace Material Specifications.
AMS 6345 (Reference 3) and AMS 6350 (Reference 4) are used where welding and moderate tensile properties are required.
AMS 6351 (Reference 5) is used where deep drawing and forming are required.
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 26 August 1998