welding chromoly to mild.....any issues?

[manta22]

Earlier in this topic it was mentioned that chrome- moly tubing was "4140". If I remember right, 4140 is used for heavy sections while 4130 is for lighter sections such as tubing.

Maybe that was only a typo.

[38Chevy454]

Earlier in this topic it was mentioned that chrome- moly tubing was "4140". If I remember right, 4140 is used for heavy sections while 4130 is for lighter sections such as tubing.

Maybe that was only a typo.

More metallurgist information.  Steel is basically classified by the first two digits and then the nominal carbon content for the last two.  So for 4130, the 41 = chrome and molybdenum as the main alloying elements; and 30 = .30 weight percent carbon.  4140 is also 41 = the 41 = chrome and molybdenum as the main alloying elements; and 40 = .40 weight percent carbon.  So the only difference between the two is that one is slightly higher carbon content.

Now you may also have heard of 4340 as a steel, the 43 = nickel, chrome and moly, and the 40 = .40 % carbon.

Most mild steel is like 1010, 1018 or something like that.  the first 10 = plain iron, no specific alloying elements; and the second 10 or 18 = .10 or .18 % carbon.

Lower carbon is better for welding and forming.  Higher carbon content is better for strength.  So much for today's quick lesson.

[manta22]

Higher carbon steels are stronger but also more brittle. Better to use an alloy steel if you need strength and toughness.

Regards, Neil   Tucson, AZ

[Rick Byrnes]

OH DEJAVOUS,
 Met 101 all over again.
next we will looking at iron carbon phase diagrams.
What a wonderful place this is....

I'm still using 4130N and heat treating very carefully....

Rick

[38Chevy454]

Higher carbon steels are stronger but also more brittle. Better to use an alloy steel if you need strength and toughness.

Regards, Neil   Tucson, AZ

Yes, the alloying elements typically add toughness while maintaining or increasing strength.  All this discussion is based on solution strengthening, and not for heat treating.  Solution strengthening means strtength you get in the normalized or non-heat treated condition.

BTW, nickel is better for toughness vs chrome or moly.  So if you want real tough material (toughness defined as the amount of energy absorbed by the metal during failure), the 4340 is better than 4140 for example.  Either 4130, 4140, or 4340 are better and tougher than a plain carbon mild steel.

Carbon content controls and defines the ultimate strength potential of the steel when heat treated, but has relatively minor effect on non-heat treated steel strength.

[SPARKY]

What a joy to get such wide coverage for such an important part of our LSR world!!!!!!! Thanks  again Willie and to all who posted!!!!!!

[akk]

Thought I might stir this up a bit.

When chromemoly 4130 is welded with enough heat to prevent rapid quenching of the heat effected zone, it is normalized or even annealed in the heat affected zone, if enough heat is applied. In this region the strength can be less than mild steel DOM "work hardened" tube in tension.

However, the structure may far exceed the strength of mild steel DOM tubing!

Sometimes the failure mode is from compressive stresses! Long tubes fail from buckling and high yield strength in the middle of the tube resists buckling. Yes the heat affected zone, if not brittle, has a lower yield strength. If the load is tensile the yield strength will be the limit, however if the tube is long and the load is compressive, high yield strength chromemoly tube will allow the tube to carry significantly larger compressive loads than mild steel tubes.

Akk

[Interested Observer]

Ref:  "Sometimes the failure mode is from compressive stresses! Long tubes fail from buckling and high yield strength in the middle of the tube resists buckling. Yes the heat affected zone, if not brittle, has a lower yield strength. If the load is tensile the yield strength will be the limit, however if the tube is long and the load is compressive, high yield strength chromemoly tube will allow the tube to carry significantly larger compressive loads than mild steel tubes. "
____________________



Not so!  Column buckling of slender members (Euler buckling) is an elastic failure mode and is determined by the stiffness of the material and the cross-sectional geometry.  Material strength has no bearing on the critical load to produce buckling.

Therefore, in the postulated situation of the 4130 tube with mild steel properties at the ends under compressive loading, one of two things may happen.  If too short to fail by buckling it will fail by local crippling at the ends at a compressive stress equal to the mild steel yield strength.  If long enough to fail by buckling before yielding in compression at the ends, the buckling load will be the same for either 4130 or mild steel, since the elastic properties (Young’s modulus) of the two steels are essentially the same.

The upshot being that 4130 does not offer increased buckling load performance.  (However, after it has buckled, it may absorb slightly more energy as it collapses).

If the member is short enough to not be subject to buckling and the 4130 is properly heat treated after welding to enhance its strength, the direct compression failure load would be improved over the mild steel version.

[akk]

I guess it gets down to..... how long is long? The technical description of length is "slenderness ratio". This ratio is calculated and depends on end conditions, length of tube, section modulus and end area.

There are two buckeling stress equations generally used Euler and Johnson. The Euler is for high slenderness ratio "long" tubes and is geometry driven, the equation does not include material strength. The Euler equation is for low slenderness ratio "shorter" tubes, the equation includes material yield strength.

In any case a compressively loaded tube with a length more than a few diameters can fail in compression at a stress less than yield.

Depending on end conditions (fixed "welded", pinned on one or both ends), slenderness ratio, material stiffness and yield strength, one or the other of these equations is used.

A higher yield strength tube can be longer than a lower strength tube before buckling occurs (all other things being equal). A higher yield strength tube can carry more load than a lower yield strength tube if the length is less than the length associated with the critical slenderness ratio.

The heat affected zone can reduce the yield strength of the tube material to the annealed condition say 35,000 psi. In the case where a high strength chromemoly tube is weakened on the ends, up to the critical slenderness ratio it can carry the same load in compression and tension.

For 1.25 x .135 wall 70,000 psi yield strength steel tube with welded ends, the critical slenderness ratio is 92 and the length at this ratio is 36 inches. This 4130 tube will have compressive strength equal to the tension strength of the heat affected zone with a load of 17,087 lbs.

A 1.25  x .135 wall 50000 psi yield strength tube with welded ends of the same dimensions could hold only 15680 lbs.compressively!

A 1.25 x .135 wall 35000 psi yield strength tube with welded ends of the same dimensions could only hold 12,807 lbs compressively!

A shorter tube will yield the the heat affected zone in compression and act as a hinge joint at the end of the tube. This hinge will cut the critical length by half.

The high yield tube at a length shorter than the length associated with the critical slenderness ratio, will be more resistant to buckling failure than the lower yield tube.

Unless I punched the numbers wrong Yield strength matters...so does length...

Akk

 

[Interested Observer]

The Johnson formula is merely a convenient contrivance and approximation that applies only to the short and intermediate slenderness ratio range.  It contains the yield strength in order to have the endpoint and tangent point of the curve located where they are postulated to belong.  It is used in lieu of the more laborious but more properly founded “generalized Euler formula” or the “tangent modulus formula.”  Inasmuch as Johnson was artificially created using yield strength, it is not surprising that yield strength can be said to influence the capacity.  Johnson may be a useful device, but is strength really the controlling quality?

It remains that the earlier unqualified statement  “if the tube is long and the load is compressive, high yield strength chromemoly tube will allow the tube to carry significantly larger compressive loads than mild steel tubes” is, in general, inaccurate and misleading.

[Constant Kinetics]

309 is specifically designed for welding dissimilar metals and is most commonly used for joining stainless and mild steel. I'm not sure how well it works or cro-moly, but it has a high nickel content and is rated to 70,000 psi tenile strength. Esab has an 800 number for these kind of questions, but they will probably recomend the most expensive product they carry when there could be something more affordable that will work just fine. ex.- they recomended an ER82 filler metal to me that costs $46 pre lb. and is sold in 30lb. in its smallest quantity when 309 worked perfectly and costs $<6 per lb. and comes in 5 and 10 lb rolls.