Chromizing-Boronizing 4

[Steve898]

What effect on UTS and fatigue properties of heat treated steel(50 HRC)does chromizing have? I believe the steps are hard chromize, harden, then temper to 50 HRC. The alloy is either plain carbon steel or low alloy carbon steel. I am curious about boronizing as well.

 

[Goahead]

I am afraid I am unable to fully understand the question.
Plain carbon steel or low alloy steel would not harden to 50 HRC. By "hard chromize" do you mean applying hard chromium electroplating? If yes, you would not heat treat after chromium plating. If you mean the surface application and diffusion of chromium powder by heat treatment, you would get a somewhat corrosion protective surface layer, not hardness.

http://www.welding-advisers.com/

 

[Steve898]

Goahead,

I appreciate your response. Perhaps I need to add some information for clarity.

I meant small section size 1/4" C1050 or similar. The chromized diffusion layer I have seen (not plated) has a hardness of over 1000HV0.1!

The core hardness was 49-51 HRC equivalent below the diffusion layer. The diffusion layer was ~0.001".

I found some information on bodykote's website about this, and I am looking for more info from other sources. Thanks.

 

[TVP]

Steve898,

If I understand your question correctly, you are describing the process of induction hardening the surface of a suitable steel (SAE 1035 and 1045 are frequently used in North America for this purpose) and then applying hard chromium plating by an electrodeposition process. The process is usually harden first, then chrome plate, then temper. The tempering operation also serves as an embrittlement relief (hydrogen embrittlement). 50 HRC on the surface is possible after low temperature tempering. This can be achieved through furnace tempering or induction tempering.

Now, to answer your question on the effect of chrome plating on UTS and fatigue, it is generally accepted that there is no reduction in tensile strength due to plating. The chrome plating is almost always polished to a very low surface roughness (~ Rz 1), which means that the surface condition is favorable. However, depending on the exact nature of the plating (thickness, type of deposit, presence of cracks), fatigue strength can be reduced. The best plating lines used today deposit a very uniform chrome layer, with many overlapping "platelets". So instead of depositing a single layer that has substantial residual tensile stress, and therefore will crack through its entire thickness, this rapid plating technology deposits a chrome layer that does not have substantial residual tensile stresses, and therefore its effect on fatigue is minimal.

 

[metengr]

Chromizing can be distinguished as either "hard" or "soft" depending on the carbon content of the substrate. For carbon contents in excess of 0.3%, chromizing can result in a surface hardening effect because of the formation of chromium carbides. The surface hardness will increase, however at the expense of local decarburization in the steel substrate.

"Soft" chromizing occurs in low carbon steels because of the lower carbon content. Instead of hardening, you have more chromium dissolved in the surface of the steel increasing corrosion resistance.

Borizing is performed using a bulk diffusion process to increase surface hardness and reduce wear. Borizing is limited to tool steels, nickel and cobalt-base alloys and cast iron that can produce intermetallic compounds with boron to increase surface hardness.

As with any surface heat treatment process, the fatigue strength of the component will be affected by tensile strength. In this case, increasing the surface hardness of the substrate using a "hard" chromizing process will increase tensile strength (locally at the surface) and thus increase fatigue strength. It is important to understand that this is a localized phenomenon and should not be counted for in the bulk properties of the component.

 

[metengr]

Forgot to mention, with "hard" chromizing the component can be heat treated after application of the diffusion coating. In this case, the fatigue strength of the component will be dictated by the tensile strength obtained from heat treatment, and not from the surface treatment.

 

[Steve898]

Thanks to all for the replies.

Metengr: What do you think the response of 7140 nitriding steel would be to the borizing or chromizing process?

 

[unclesyd]

I don't know about 7140 steel but we Chromized tons of 4140 machined parts with no problems prior to introduction of the PH Stainless Steels.

 

[metengr]

The 71XX, 72XX are tungsten-chromium tool steels. I don’t have any real experience with these steels to offer any advice on their response to chromizing. What are you looking to obtain?

I would ask Bodycote for advice on response to chromizing or borizing.
The web site below contains an interesting paper concerning the affects of various surface treatments on tribological properties;

http://www.ctd.uni-lj.si/publications/bojan/Vacuum

68 (2002).pdf
PII: S0042-207X(02)00282-8

 

[Steve898]

I want what everyone wants: Increased strength, increased wear strength, high toughness, and low cost.

I liked the structure I described earlier, I believe it will produce very good wear strength with no sacrifice to toughness, fatigue, or UTS.

I was unable to open the website, what is the name of the paper?

Thanks for your help.

 

[metengr]

The title of the paper is;
"Tribology of thin films and their use in the field of machine elements" by Bojan Podgornik and Jose Visintin

in Volume 68 of the journal "Vacuum", Surface Engineering, Surface Instrumentation and Vacuum Technology

 

[NJmetallist]

Boronizing gives much higher corrosion and wear resistance then chromizing since forms deeply diffused FeB Fe2B crystal-like embedded coating. Impossible to peel off like nickel or chrome electoplating.

The material to be boronized must be raw, non heat treated.
Results on Inconel 718 are not as great as iron alloys since the high nickel content. If I have to make a guess about 7140 nitriding steel, boronizing will form good wear-resistance coating with lesser cost then chromizing. FeB coating is somewhat brittle.

I am performing Boronizing research work on various alloys including W-, Ti-, Ta-, Zr- based but mostly work with 1018 and 4340.

 

[Steve898]

Njmetallist: I have had some 1022B (kinda like 1018) pins boronized, then I hardened and tempered them. No metallurgical results yet. I haven't tried the 7140. Thank you for your response.