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WELDING “HEAT TREATED”  4130

(QUENCHED AND TEMPERED) Chrome Moly (Chromoly)

cr-moPronouncing Chrome-Moly
I always pronounce it as Chrome-Moly.  Not, cromoly, pronounced like two animals!
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Video-Welding 4130 CrMoly

 

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This web page presents information about 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 heat treated,  high strength 4130 tubing  without subsequent appropriate heat treatment (meaning heating the finished weldment to 1600 F, quenching in oil or water and then tempering to 1000 to 1150 F) 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  issues with:  

  1. AA Fuel Dragsters being allowed to use higher strength, heat treated 4130 and increased stresses for some frame members. (Cory McClenathan crashing at Bristol, Tenn. apparently because of excessively thin wall frame tubes that experienced local buckling at 300 mph!)
  2. Some Funny Cars where Heat Treated 4130 apparently had been used and the chassis broke in what reportedly appears to be a brittle manner.
  3. Questions received in technical talks at American Welding Society meetings and emailed to our Web Site from race car and custom motorcycle frame builders wanting to use this higher strength -"stiffer" material (when it is NOT stiffer!)
  4. A recent article in a popular car magazine which reinforced the need to define that increased “strength Does Not Meanincreased 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.

Note: If your going to "heat treat" the finished welded 4130 part (by heating to 1600 F, quenching in oil or water and then tempering to about 1100 F) see comments from my original Article published in The American Welding Societies Journal and quoted on this web site page.

WHY 4130 IS STRONGER WHEN “HEAT TREATED”

Also, Why Recommended Welding Rod and Procedures for Welding Normalized 4130 Cannot be Used to Solve the  Problems That Can Occur When Joining Heat Treated Material.

In the mid 1970’s, while managing a welding shielding gas, MIG wire, TIG rod R&D Laboratory, I received a call from a dragster chassis manufacturer requesting a recommended TIG rod for joining normalized  4130.  They did not plan to use preheat or any post weld heat treatment. To minimize the chances of weld cracking when mixed with this high carbon (by welding standards) 4130 material, a low carbon AWS ER70S-2 TIG rod was recommended. This alloy rod 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 cubic arrangement (right in the figure above) and carbon atoms are interspersed between.  If cooled slowly, the atoms will rearrange to a low stressed Body Centered cubic state (left in figure.)  However if quickly quenched from 1600 F the structure does not realign to a Body Centered  cubic structure but rather a Body Centered Tetragonal arrangement (as noted in accompanying figure one side of the rectangle is shorter than the other so it is not a cube) leaving a very highly stressed material which is very hard and strong.  This metallurgical structure is called Martensite (named after metallurgist Adolf Martens.)  The resulting  tensile strength could be in the range of 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 sharp 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 valuable to understand the Charpy test.)  The photo below left shows two broken Charpy test specimens.  The lower two matching pieces  shows how brittle steel looks. There was little energy required to initiate and sustain the fracture. The upper broken specimen shows the deformation that took place at the outer edges and the matt finish in the center requiring more energy to break the Charpy bar.    To reduce this brittleness, the quenched and hardened steel can be tempered.  Tempering requires reheating the very quickly cooled steel to usually 700 to 1100 degs F.  For gears and items that need to be hard, and that will operate at higher temperatures, a low tempering temperature may provide enough toughness.  However for 4130 used in structures, the tempering is usually done at about 1000 to 1100 degs. F to provide acceptable 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  than the as quenched material.

There is a tempering temperature range between 500 and 700 degrees where the steel may not improve and may even reduce somewhat in toughness.  This reheat range should be avoided (see graph below.)

 In the weld Heat Affected Zone (HAZ) we can't avoid reheating the tubing from a temperature close to the melting point of the steel, 2500 deg F, to temperatures in and below that range which will decrease the strength!  Tempered properties are also dependent on tempering time.  The following table presents the strength of 4130 after various heat treatments:

Note the strength  can be reduced after heat treating to 115 ksi (Note 1 ksi = 1000 psi) by carefully reheating to 1200 F.  The exact temperature and time are very important.  Heating with a torch can not assure the proper desired properties. 

The graph left shows the resulting toughness  after tempering to specific temperatures.  Note there is a low tempering temperature where the toughness does not increase and may actually decrease.

CAUTION: When welding heat treated, Q&T 4130 the area adjacent to the weld deposit (Heat Affected Zone) will reduce in strength.

 

WELD HEAT AFFECTED ZONE PROPERTIES

Weld metal chemistry can be controlled by the type of welding filler rod/wire used.  The amount of base material melted into the weld deposit must also be considered.  When very small fillet welds are made in 4130 tubing the large amount of melted high carbon (by welding standards) base material in the weld deposit can significantly increase weld strength.  That is why we recommended AWS ER70S-2 when welding normalized  4130.  It has very low,  0.06 carbon so when mixed with the 0.33 carbon in the 4130 the weld deposit carbon content is at a lower level, less susceptible to cracking.

Where the melted weld properties are influenced by the rod/wire selection, they have no influence in the area next to the weld called the Heat Affected Zone (HAZ.)  In the photo left, the weld and HAZ are revealed by chemical “etching” a cross section.   The outer etching boundary is defined as a line that reached about 1300 degrees F.  The line exists because a metallurgical change occurs in steel at that temperature.  Therefore the visible HAZ  was heated to a temperature from about the melting point of the steel (about 2500 degs F) next to the weld through this 1300 deg F line then on to 500 F and lower. The width of the HAZ  area will be larger when welding thinner materials.   If previously quenched 4130 is tempered to 1050 degs F to achieve the desired properties, we have essentially tempered the tubing next to the weld from 2500 degs F all the way down to the 1050 degs F!  The time at temperature is also a factor and will depend on weld heat input, wall thickness etc.  In general, the HAZ  will have a strength much less than the “after tempering” 150,000 psi, for example!  In fact that area will probably be no stronger than if we welded the more typically used normalized 4130 tubing.    Normalized  4130 tubing was cooled more slowly from a high temperature, never creating the hard, brittle Martensitic structure and it is also much tougher than the as-quenched material Normalized  4130 usually has a strength of about 95,000 psi.  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 as shown in the photo right.  Note the area that reached 1000 deg F is further away from the weld than the visible 1300 F outer etching bounder. 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 lower strength 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 (with some potentially brittle areas) HAZ!

An interesting way to see what temperatures are reached when a  weld was made is on a sand blasted surface is shown in the above photo.  Steel oxidizes to specific colors depending on the temperature reached.  These are called temper colors.  This photo shows how far from the weld the plate reached 500 F while welding.

STIFFNESS

Why use “high strength” 4130 in the first place?  All else being equal 250,000 psi oil/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 or what is referred to as yield.)  This assumes the same tubing diameter and thickness are being compared.  Generally when switching from a lower strength material, for example mild steel at 50,000 psi to 4130 at 95,000 psi tensile strength, a different size tube will be employed.  Refer to the web page of properties and metallurgical definitions under the section on 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 and 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 lower stress.  Other joining techniques can also be considered.  Racing bicycle manufactures use special brackets or "Lugs" that capture the tubing and then they braze the joints at much lower temperatures than caused by welding.  You could also heat treat the final product.  It is very difficult to take a complex fabricated chassis, heat the assembly (with the needed fixture  to minimize distortion) to 1600 deg F, then quench in oil and have it remain in the desired shape!  However one Baja race car team reports they were going to do just that.  Not sure how they managed the inevitable distortion or if it was a successful approach!

SIDE BAR

An interesting analysis was developed from information presented on Metallurgical Technologies' web site.  They examined a weld made in 4140 steel which has 0.40 carbon.  This steel is more hardenenable than 4130 meaning at typical TIG welding cooling rates it can produce 100% hard, brittle Martensite in parts of the HAZ.  This is what appears to have occurred in this weld. Two microhardness traverses of the Weld, HAZ and Base Metal are presented in the above graph (typically these are taken near the top and center of a welded section.).  Converting the hardness data shown on the black line to approximate tensile strength; the unaffected Base Metal was ~150 ksi tensile strength and the weld much harder with a equitant tensile of ~240 ksi.  The HAZ next to the weld had a hardness equivalent to ~300 ksi.  The base material may have been quenched and then tempered to about 1100 F to achieve 150 ksi tensile strength.  Mechanical property data for 150 ksi Q&T 4140 shows it may have a reasonable Charpy toughness of ~60 ft-lbs.  When tempered to only 400 F it will have a tensile of ~260 ksi and only a brittle, 10 ft-lbs Charpy impact toughness.  The ~300 ksi HAZ toughness would be even lower -"glass like!"    In addition, the hardness traverse shown on the  red line indicates a dip in strength to the equivalent of 110 ksi.  This probably occurred in an area that was subjected to a slower cooling rate and reached a temperature above the original tempering temperature. To reduce the HAZ strength and improve ductility in this 4140 weld, a minimum preheat of 400 F or higher should be used even for thin material to lower the cooling rates sufficiently to avoid the formation of Martensite.

Note, 4140 steel is about twice as hardenable as 4130.  This means a weld cooling rate that is sufficiently slow to just avoid producing Martensite in a 4130 HAZ can produce a 100% Martensitic structure in the HAZ of 4140 steel (See Note Below.)   With its lower, 0.30 carbon 4130  is less likely to have a brittle HAZ but if welding heat treated, higher strength 4130  the HAZ will decrease in strength.     Although 4130 presents less of a HAZ problem then 4140 when welding Normalized material, if welding higher strength, heat treated 4130 a hardness traverse will define the approximate amount of strength reduction and is a method to evaluate weld procedures and HAZ properties. 

The weld macro section shown on the left of the above figure is not from their web site but shows how a hardness traverse is made.  A hardness testing device incorporating a microscope (one type shown in white) is used employing a small diamond indenter.   The small black dots in the polished cross section (seen between the white arrows) are the very small indentations made with the hardness tester.  The magnified photo at the bottom shows their shape.  The depth of the indent with a fixed load is a measurement of  hardness.

My recommendation is to have some testing done by a competent metallurgical or welding research firm to evaluate the quality of weldments in 4130.  With the amount of sponsor money spent on most racing series they surely can be called upon to assist with researching weld quality and materials suitability issues.

 Email for recommendations on hiring a firm. 

 

Note: As seen in a Continuous Cooling Diagram of 4130 (Click to See) the nose of the graph is very close to TIG weld cooling rates in thin material.  With cooling rates that reach 900 F after about 5 seconds little if any Martensite will be produced.  With 4140 the nose of the curve will be displaced right to about 9 seconds.  Even the slower TIG weld cooling rate shown will produce Martensite in the HAZ.  The cooling rate curves were obtained using equations from the AWS Handbook;  See Calculations.

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If higher strength steel is felt necessary why not use an alloy that is strong, tough and more readily weldable like HY-130. 

For More Information on Welding 4130 Chrome Moly:

 

Click for BASIC 4130 WELDING DETAILS

Click for TECHNICAL & METALLURGICAL DETAILS

Click for  EQUATIONS Defining Weld Cooling Rate in Tubing

Click for METALLURGICAL DEFINITIONS

Click for DEFINING "EFFECTIVE HEAT INPUT"

Click for WELDING A BETTER STEEL;  HY 130

 

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