Nitriding v Carburizing 2

[kenre]

I need to machine and harden some small 2 stroke engine crankshafts. Used in very high stress and rpm conditions.

Which is the better option for this? material choice can be made to suit.

Shafts will run in normal c3 clearance ball bearings.

Ken

 

[sreid]

I'm not an expert on this but you might need both. I think that flame carburizing is used to add carbon to the surface layer of bearing races (case hardening). Nitriding, I believe, adds Nitrogen to the surface layer to put the surface layer in compression to better resist fatigue failure (crank pin fillets).

 

[metengr]

See faq330-1075

 

[kenre]

Thanks metengr,

best description i have seen yet.

Ken

 

[swall]

One of the things you need to consider is that with a nitrided case, you will have a core material that will only be as hard as can be obtained with a 1000F temper. With many low alloy steels that can be nitrided, such as Nitralloy 135 or 4140, this means the low Rc 30's range. In applications involving high contact stress, this hardness is too low to properly support the nitride case. So, the answer to your question will be influenced by your choice of material for the cranks.

 

[kenre]

Thanks swall, after reading the link i had seen this prob.

If i was to have them carburised, with a higher core strength, would this reduce bending to any degree? The crank runs in very close tolerance with the case to perform sealing with the oil film present. Closer the better for performance.

Ken

 

[TVP]

kenre,

When you say "would this reduce bending" in your last post, do you mean distortion due to the carburizing and quenching process? Or do you mean a reduction in bending strength due to a higher core hardness?

Yamaha has started using Supercarbonitriding for crankshafts on the YZ450F motorcyle, due to the use needle roller bearings at the big end of the connecting rod, and the resultant high Hertzian contact stresses (up to 3 GPa). They call the process Supercarbonitriding because there are three distinct phases that occur prior to tempering:

1. Carburizing at 930 C
2. Reduce temperature to 850 C followed by oil quench
3. Re-heat to 820 C to modify carbide structure & reduce austenite grain size (also introduce NH₃ to begin nitriding) followed by oil quench
4. Low temperature tempering (190 C)

Using this process and SCM420 alloy steel (similar to SAE 4120), the surface hardness was ~ 750 HV (~ 62 HRC), the core hardness ~ 400 HV (~ 40 HRC), approximately 28% retained austenite, and a residual compressive stress at the surface of 630 MPa. Rolling contact fatigue life was improved vs. carburizing by 1.6 times. The details are described in SAE Technical Paper 2003-01-0916 "Heat Treatments to Improve the Rolling-Contact Fatigue Life for Crank Pin of Motorcycles". The author has also recently written a book entitled The science and technology of materials in automotive engines (Woodhead Publishing and CRC Press, 2005) which desribes all of the components of modern engines (2-stroke, 4-stroke, gasoline, and diesel) and the materials and manufacturing processes used. I highly recommend it.

Another good paper on high performance crankshafts is SAE Technical Paper 942517 "The Design Considerations, Design Methodology and Materials of the Lamborghini 3.5 L Formula 1 Engine Crankshaft". They used gas nitriding along with 3 steel alloys: Supernitralloy (0.22C, 5Ni, 0.5Cr, 0.25Mo, 2Al, 0.1V) AFNOR 32CDV13 (0.3C, 3Cr, 1Mo, 0.2V), and AFNOR 40CDV20 (0.4C, 5Cr, 1.3Mo, 0.45V, 1Si). The Supernitralloy was quenched from 900 C, solution treated at 690 C, aged at 560 C, and then nitrided at 530 C. This produced a core hardness of 44-46 HRC, case hardness of 970 HV (> 68 HRC), with a total nitided thickness of 0.55 mm. 32CDV13 was processed to 36-38 HRC, case hardness of 850 HV, and nitride thickness of 0.70 mm. This option provided much better toughness than the Supernitralloy. 40CDV20 was processed much like tool steels are, meaning multiple heating and cooling throughout the process to optimize residual stresses and microstructure. The final process was rough machining-->annealing at 830C-->second rough machining-->stress relieving at 800 C-->first finishing-->austenitizing at 990 C then quenched-->series of tempering treatments between 570 C and 610 C. The last 0.5 mm was removed after the last tempering. Core hardness was 44-48 HRC. Nitriding was performed for 90-100 hours to produce case hardness of 1050 HV and thickness of 0.6 mm.

The papers and book can be obtained by using the following links:

http://www.sae.org/servlets/techtrack?PROD_TYP=PAPER

http://www.woodheadpublishing.com/en/book.aspx?bookID=639

 

[kenre]

TVP, bending under operation. if i can reduce crank bending, performance and longevity will be better. Yes these engines do flex the short cranks.

They will be machined, hardened and then finished ground.



Have been looking at 8620.....

Another material is V136 (hy-tuf) have been unable to find much info on this tho

 

[TVP]

kenre,

The bending strength should be better after carburizing than with nitriding, since the case depth is usually better (> 1.0 mm for carburizing instead of 0.5-0.7 mm for nitriding). Core hardness is usually higher as well, especially if newer alloys beyond 8620 & 9310 are considered (secondary hardening produces core hardness > 45 HRC). Both of these will result in increased bending strength. However, it sounds like you may be concerned with deflection under bending, which is a geometry/stiffness issue, not a microstructure and strength issue. Static and dynamic stiffness will not be appreciably affected by either case hardening treatment.