Case hardened cylinder bores?

[Robby Ackerman]

Over the weekend I mentioned to a friend that I had the block in my 190SL sleeved because the metal in the sleeves is harder than the metal of the old '50s block, and I could expect a longer life from the sleeved block.

He asked if I could have had the cylinder bores case hardened. He said he thought cylinder bores of the '50s were treated with nitric acid to harden them and wondered if I could have had my bored block treated in a similar way to harden it.

I've never heard of this. Can someone shed some light on this subject?

[Walt Puryear]

Robbie,

I consulted my '64 edition of the American Society for Metals Metals Handbook. On the subject of case hardening, there were nine chapters as briefly described below. More than you wanted, I'm sure, but there was no mention of nitric acid.

Gas Carburizing – a high carbon gas such as propane or natural gas is circulated with furnace gases to supply carbon for absorption and diffusion into steel.

Pack Carburizing – carbon monixide is derived from a solid carburizing compound by reaction within the carburizing container. Carbon Dioxide is the byproduct

Carbonitriding – is a case hardening process in which carbon and alloy steels are held at an elevated temperatures in a gaseous atmosphere from which they absorb caron and nitrogen simultaneously, and are then cooled to room temperature at a rate that will produce desired case and core properties. The carbon is derived from any carbon rich gas and ammonia is the source of nitrogen.

Cyaniding – or liquid cabonitriding, is a heat treating process that produces a file hard wear resistant surface on ferrous parts. Steel is heated in a suitable bath of molten cyanide salt, the steel surface absorbs both carbon and nitrogen. Hardening depths are up to .10”

Liquid Carburizing – same as cyaniding but with different percentages of carbon and nitrogen to produce hardened depths up to .25”

Liquid Nitriding – cyaniding at temperatures below the transformation temperature, also adds more nitrogen and less carbon.

Gas Nitriding – case hardening processs whereby nitrogen is introduced into the surface of a solid ferrous allow by holding the metal at suitable temperature in contact with a nitrogenous gas, usually ammonia. Quenching is not required. Temp is between 925 and 1050 deg.

Flame Hardening - a ferrous allow is rabidly heated above the transformation temperature by direct impingement and then cooled at a rate to produce the hardness and other properties desired.

Induction Hardening - same a flame hardening but heat is supplied by electromagnetic induction.

Walt

[ja17]

Hello Robby,

I do not believe the cylinder bores on these engines were ever surface hardened. The crankshafts on the 300SLs were nitride surface hardened and most of the camshafts and rocker arms on these early Mercedes engines were nitride surface hardened I believe.

[Robby Ackerman]

Joe

Thanks. How is the Adelaidie 450SL?

[ja17]

Hello Robby,

The Adelaide 450SLC has been sitting quietly in the shop. I am planning on doing some suspension work on it this summer. Springs, shocks etc. Would still like to make it to Targa Newfoundland some day.

Happy motoring!