Exam review

http://blog.eurotherm.com/blog/2010/07/08/what-is-the-cost-difference-between-carburizing-and-nitriding-2/:

The graph illustrates that some Nitrided steels have a higher surface hardness than Carburized steels but a lower overall total case-depth. The higher hardness comes from combining Nitrogen and alloying elements in the steel and usually for straight Nitriding you would use what is termed as an alloyed steel. For Carburized steels you would normally use a lower alloy steel (unless Vacuum or Low Pressure carburizing). 

For basic components that require some improvement in material properties, generally the lower cost treatments are to harden and temper (through harden) or carburize to shallow depths. However,  engineered components require more sophisticated processing that may result in additional costs but as previously explained this is more than offset by the increase in the material properties that result from the treatment. - See more at: http://blog.eurotherm.com/blog/2010/07/08/what-is-the-cost-difference-between-carburizing-and-nitriding-2/#sthash.1MEawtm9.dpuf

 Typically the higher the alloy content of the steel, the higher the cost of the base material. The Nitriding process CAN be a more costly process to run based on batch processing and long cycle times. However, other benefits of the Nitrided process such as lower temperature giving less distortion, additional strength properties from an alloyed steel, higher hardness (can lead to higher wear resistance) and other material properties can help to offset these costs.

• See more at: http://blog.eurotherm.com/blog/2010/07/08/what-is-the-cost-difference-between-carburizing-and-nitriding-2/#sthash.1MEawtm9.dpuf

http://heattreatment.linde.com/international/web/lg/ht/like35lght.nsf/repositorybyalias/wp_cbrzg_10/$file/10.pdf:

Carbonitriding can be applied to low cost, low alloy steels. The com- bination of adding nitrogen as well as carbon to the case increases the case hardenability sufficiently to result in a martensitic case that would not be possible with pure carburizing.

http://www.twi-global.com/technical-knowledge/faqs/process-faqs/faq-what-is-carburising-carbonitriding/:

All three processes rely on the transformation of austenite into martensite on quenching. The increase in carbon content at the surface must be high enough to give a martensitic layer with sufficient hardness, typically 700HV, to provide a wear-resistant surface. The required carbon content at the surface after diffusion is usually 0.8 to 1.0%C. These processes can be carried out on a wide range of plain carbon steels, alloy steels and cast irons where the bulk carbon content is a maximum of 0.4% and usually less than 0.25%. Incorrect heat treatment can lead to oxidation or de-carburisation. Although a relatively slow process, carburising can be used as a continuous process and is suitable for high volume, surface hardening.

Carbonitriding is undertaken on a similar range of steels although the bulk carbon content can be as high as 0.4 to 0.5%. The process is particularly suited for hardening the surface of components that need a through-hardened core, such as gears and shafts. Carbonitriding is a modification of gas carburisation where ammonia is added to the methane or propane and is the source of nitrogen.

http://www.onviollc.com/email/onvio_ontech/ontech_0906_v2a.html:

Which heat treatment method is better for precision gears?

Both carburizing and nitriding are acceptable heat treating methods for precision gearing. Through engineering, development and testing, Onvio has determined that carburized gears offer critical advantages for precision gearboxes.

HARDNESS DEPTH & CORE STRENGTH
Carburized gears feature a significantly deeper hardened layer whose hardness gradually decreases to the core hardness. This structure provides superior surface contact fatigue properties and ductility/impact resistance and strength of the core. In addition sufficient case depth obtained in the carburizing process is required as a needle roller bearing surface in the planet gears.

Nitrided gears can have a “white layer” on the surface which is very hard and brittle. If is not removed it can be prone to flaking and cracking leading to heavy surface fracture and gear failure. The hardened case depth is significantly thinner than in similar carburized gears and transitions to the core hardness immediately behind the case.

SURFACE HARDNESS
While both technologies offer very good surface hardness, Onvio believes that the depth penetration of carburizing provides our customers with superior performance and reliability for their demanding applications.

http://www.citysteelht.com/surface_treatments.html:

SURFACE HARDENING – INDUCTION PROCESS

It is frequently desirable to harden only the surface of steels by simply changing their microstructure without altering the chemical composition of the surface layers.  If steel contains sufficient carbon to respond to hardening, it is possible to harden the surface layers only by very rapid heating for a short period of time, thus conditioning the surface for hardening by quenching.

 

Hardenability—ability of steel to form martensite when quenched.

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Case hardening Techniques compared