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WELDING “HEAT TREATED”  4130

(QUENCHED AND TEMPERED) Chrome Moly

This information relates to welding heat treated, high strength 4130 Chrome Moly tubing for race car frames where no post weld heat treatment is utilized.  We are not advocating this be done, in fact in most cases just the opposite. 

The reason for discouraging welding high strength 4130 tubing without subsequent appropriate heat treatment is that the area next to the weld will have much lower strength and may contain some brittle areas!

We’re presenting this information because of recent issues with:  

1. AA Fuel Dragsters (Photo Above, Cory McClenathan Crashes at Bristol, Tenn.) and Funny Cars where this material was (is) used;
2. Questions received in technical talks at American Welding Society meetings and those received on our Web Site from race car and custom motorcycle frame builders;
3. A recent article in a popular car magazine which reinforced the need to define that increased “strength” Does Not Mean “increased stiffness!”

A Definitions Page is provided to keep this discussion of reasonable length.  Just click on the words in bold italics to obtain a non metallurgist definition.

WHY 4130 IS STRONGER WHEN “HEAT TREATED” AND

WHY STANDARD RECOMMENDED WELDING ROD AND PROCEDURES FOR WELDING NORMALIZED 4130 CAN NOT BE USED.

In the mid 1970’s while managing a welding wire/rod R&D facility I recommended to a dragster chassis manufacturer that AWS ER70S-2 welding rod be used when welding normalized 4130.  For race car frames they did not plan to use preheat or post weld heat treatment.  This rod/wire has become widely accepted as an excellent solution and has proven successful over many years.  Welding heat treated  4130 has totally different implications.  The area next to the weld not just the weld metal becomes the critical issue. 

The reference to heat treated steel generally means the material is heated to a temperature where a structural change occurs in the crystal lattice.  In the case of 4130 about 1600  degs F.  At these temperatures the iron atoms have a Faced Centered arrangement ( Photo Left) and carbon atoms are interspersed between.  If cooled slowly the atoms will rearrange to their less stressed Body Centered state.  However if quickly quenched from these temperatures the structure is maintained leaving a very highly stressed material which is very hard and strong.  This metallurgical structure is called Martensite.  Tensile strength could be as high as 250,000 psi.  Unfortunately it is also very brittle.  What does brittle mean?  It means this very strong steel can act like glass when subjected to a shape blow or highly stressed environment!    A positive term used to quantify a steels resistance to brittle failure is a property called toughness. One measure of toughness is the Charpy test (although not useful for very thin material its useful to understand its use.)  The photo left is a broken Charpy specimen which shows how brittle steel looks.  Very little energy was required to initiate and sustain the fracture.  To reduce this brittleness the hardened steel can be tempered.  Tempering requires reheating the very quickly cooled steel to usually 500 to 1100 degs F.  For gears and items that need to be hard, 500 degs F may provide enough toughness.  However for 4130 used in structures the tempering is usually done at about 1000 to 1100 degs. F to provide some resistance to brittle failure.  At these tempering temperatures a fully quenched and tempered (Q&T) 4130 will have strength of about 150,000 psi but will have higher toughness and ductility. 

There is a tempering temperature range between 500 and 700 degrees where the steel may actually not improve in toughness and should be avoided (however in the weld Heat Affected Zone we can't avoid reheating the tubing from the melting point of the steel, 2500 deg F, to temperatures in and below that range!)  Tempered properties are also dependent on tempering time.  This discussion will not provide details about this other than to say when welding heat treated, Q&T 4130 it must be understand the area adjacent to the weld deposit ( Heat Affected Zone ) can have some unusual and undesirable metallurgical structures.

WELD HEAT AFFECTED ZONE PROPERTIES

Weld metal chemistry can be controlled by the type of welding wire or rod used.  The amount of base material melted into the weld deposit must also be considered.  When very small filet welds are made in 4130 tubing the large amount of melted high carbon (by welding standards) material in the weld deposit can significantly increase strength.

The properties if the weld metal can influenced by the rod/wire selection but there is little control of what happens in the area next to the weld called the Heat Affected Zone (HAZ.)  In the photo left, the HAZ is usually referred to as the etched area between the melted weld deposit (visible by the coarse grains) and the outer etching boundary which in most steels defines an area that reached about 1300 degrees F.  This line exists because a metallurgical change occurs in steel at that temperature that can be revealed by chemical “etching.”  In essence the HAZ was heated to a temperature from about the melting point of the steel (about 2500 degs F) next to the weld all the way to not heated at all.  For issues related to tempering, if the 4130 was tempered to 1050 degs F to achieve the required properties, after welding we have essentially tempered the tubing next to the weld from 2500 degs F all the way down to the 1050 degs F and lower!  The time at temperature will depend on weld heat input, wall thickness etc.  In general we will have an area much softer than the “after tempering” 150,000 psi!  In fact it will probably be no stronger than if we welded the more typically used 4130 Normalized.   Normalized  4130 tubing was cooled much more slowly from a high temperature, never creating the hard brittle Martensitic structure but is also much tougher.  Normalized  4130 is usually about 95,000 psi in strength.  The mixture of metallurgical structures present in the HAZ of welded Q&T 4130 may contain some brittle  areas.

WHAT HAPPENS WHEN WELDED Q&T 4130 IS STRESSED?

When the structure is stressed, the ~150,000 psi strength tubing material will have a much weaker (perhaps 90,000 psi)  HAZ area between the joined pieces.  The strength will be reduced in the HAZ in a band heated from the 2500 degs F area next to the weld all the way down to a band that reaches the 1050 degs F tempering temperature.  That weakened band could be wide in deposits made in  thin wall tubing.  It addition, if any brittle metallurgical constituent exists, cracks may form and propagate in the highly stressed HAZ even at lower overall stress levels.  Because of the potential brittle areas in the HAZ, our usual suggestion which has proven successful for many years of using an very ductile AWS ER70S-2 welding rod and making slightly larger fillets to compensate for the somewhat lower strength will not help a weak brittle HAZ!

STIFFNESS

Why use “high strength” 4130 in the first place?  All else being equal 250,000 psi water quenched 4130 tubing is no “stiffer” then mild steel with 50,000 psi tensile strength!   Both will bend the same amount when loaded (assuming the structure does not take a permanent set.)  This assumes the same tubing diameter and thickness - which generally is not the case.  Refer to the page of properties and metallurgical definitions under the section stiffness, to understand what needs to be done to achieve a “stiffer”  structure.  4130 Chrome Moly is also the same weight per cubic inch as mild steel.  The only way it can be stiffer or lighter is to use large diameter tubing with a thinner wall thickness.

SUGGESTIONS

What Should be Done to Join Heat Treated 4130?

First be sure it is understood why the higher strength is being used.  If there is a real structural benefit or are you just getting into and area where local buckling may occur?  If the extra strength is a real benefit, then perhaps welds can be placed in an area of low stress.  Other joining techniques can be considered.  Racing bicycle manufactures use special brackets that capture the tubing and then braze the joints at much lower temperature.  You could heat treat the final product.  Sounds difficult but one Baja race car team reports they did just that. 

My strong recommendation is to have some testing done by a competent welding research firm.  This should include a measure of resulting weld joint strength, toughness and fatigue properties. With the large amount of sponsor money spent on most racing series they surly can be called upon to assist with researching this safety issue.

 Email for recommendations 

If higher strength steel is felt necessary why not use an alloy that is strong, tough and much more readily weldable like HY-140. 

Click For Information about this steel.

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