Why Free Machining and Impact Strength Don’t Mix

November 29, 2011

 It is critical to understand that the selection of free machining steels goes against the ability of those components to withstand impact loads.

Charpy impact values are reduced by free machining additives.Impact values increase with increased hardenability.

 

Impact strength is often an important design consideration in mechanical components. Cost to manufacture is also an important consideration in mechanical components.

Free machining grades can reduce the cost to manufacture precision machined components. But free machining additives reduce the impact strength of the steel. Materials should be selected on the basis of complying with design requirements, not just low cost to manufacture.

The low carbon free machining steel grade 1215 exhibits a particularly low level of toughness over a wide range of temperatures. Even light impact loadings are a bad fit for this grade of steel. The principal effects of the free machining elements  (Sulfur and Manganese) added to this steel are to lower the upper shelf or ductile portion of the absorbed energy curve.

The effects of hardenability can be seen between the 4140/41L40 and 1141 steels. While the presence of lead in the 41L40 does drop the upper shelf energy somewhat, the biggest difference can be seen  to lie between the 1141 and the two 4140 grades. The lower hardenability of the 1141 on mill cooling in addition to the effect of the manganese sulfide additives explains this difference.

The greatest difference however that can be seen from this figure is the vast difference between the two low carbon steels, Grade 1215 and 1018. Even at 212 degrees F the upper shelf energy of the 1215 is roughly  only a third of that of grade 1018.

Rule of thumb: If a steel grade machines well- it probably has miserable charpy impact properties. 

Figure 1 taken  from The Assessment of The Mechanical behavior Of Free-Machining Steels, J.T. Berry and  R. Kumar, School of Mechanical Engineering, Georgia Institute of Technology; R.G.Kumble, Vermont American Corporation. 1975 ASM  Mechanical Working and Forming Division, International Symposium  on Machinability.

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.40 Carbon Steel Brinell Hardness vs. Tempering Temperature

October 5, 2010

Hardness of Quench and Tempered alloy steels is a function of the tempering temperature. The higher the tempering temperature, the lower the hardness.

This is called an inverse relationship.

And it’s why some people call tempering “drawing.”

The temper “draws the hardness out of the steel.”

Normalized at 1600F, Quenched in oil 1550 F, Tempered 2 hours

 These curves are a rough approximation of the as tempered brinell hardness for the grades shown. For example, I have other data for 4340 that shows 440 BHN at 800F; 410 at 900F; 380 at 1000F; 340 at 1100F, and 310 at 1200F temper temperature.

Fire! Can't do a blog on heat treat without a picture of fire.

Your mileage may vary, in other words, but this graph is close enough for ‘considered judgement.’

Additional 4140 data that I have from my notes suggests 397 BHN at 800F; 367 at 900F; 335 at 1000F; 305 at 1100F and 256 at 1200F.

If you have better data from your process – USE IT.

Better yet, if you have time, send a sample to your heat treater for a pilot study.

In the absence of data from your process, the above figure and data will give you “a place to stand” in understanding what is possible when heat treating .40 carbon alloy steels- the steels most commonly encountered in our precision machining shops for Automotive, Aerospace, Agricultural and general applications.

Here is a video from PMPA member company Nevada Heat Treating to give you an inside look at what goes on at a heat treat service provider.

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