Hardening of 18/6 Stainless Steel 3

[tld23]

Hi,
I have some components that are already made in 18/8 stainless steel. I assume that case hardening with carbon will not work because the nickel will block the carbon diffusion. Any recommendations on hardening the surface would be welcome. TERRY

 

[TVP]

Carburizing can be done, but it comes at the expesne of corrosion resistance, since the carbon combines with the chromium present to form grain boundary chromium carbides. Stress Corrosion Cracking is especially problematic.

Nitriding can be done-- gas, plasma (ion), and liquid are all possible. Consult with a local heat treater. We can provide some suggestions if you need them.

 

[mcguire]

Any introduction of carbon or nitrogen will cause chromium to be lost from solution with the attendent loss of corrosion resistance. If that is acceptable, nitride.
If you can superficially cold work the surface, say by shot peening, you can harden the surface. The degree of hardening will be a strong function of the temperature at which it occurs. Get down below 32F if possible.

 

[kenvlach]

You can also hard chrome plate or electroless nickel plate. The electroless nickel must be heat treated for maximum hardness. Specification (MIL-C-26074, AMS 2404C, ASTM B733) is for a minimum hardness of 850 Knoop (HK100).

 

[DR13]

There is a low temperature carburizing process known as Kolsterising that may be used to harden the surface of your components made in 18/8 stainless steel.

 

[kenvlach]

Have you considered PVD coatings, such as TiN?

Since Kolsterising was mentioned, I'll mention that there are many proprietary processes that make great claims, e.g.,

"The Sheradize treatment ...uses unique circuitry and a proprietary cold electrolyte which creates a molecular structural change. The new surface causes no dimensional change, the treatment is all penetration. There is no warpage since the treatment is done at sub freezing temperatures...Hardness: Non-heat treated steel generally increase 4-6 Rockwell C points..." (

http://www.lovattprocesses.com/index1.htm)

"The ROCKLINIZER equipment, using the ROCKLINIZING process, applies a hard and wear resistant surface of TUNGSTEN CARBIDE, TITANIUM CARBIDE and ROCKHARD electrode material to metals, tools and dies.
The ROCKLINIZER electronically impregnates and deposits wear-resistant material both underneath and on top of metal surfaces. Unlike welding or metal spraying, no appreciable heat is generated; and the electrode material will not separate or flake off the workpiece. After ROCKLINIZING, no heat treatment, grinding or other surface treatment is necessary." [this one is partially a surface build-up, but it could be ground down]
(

http://www.rocklinmanufacturingco.com/rocklinizer.html)

"Colored Stainless Steel:
The color is produced by an electrochemical process that creates a thin layer of chrome oxide on the stainless steel. By varying the thickness of the oxide coating, colors ranging from Champagne, Bronze, Blue, Gold to Black are achieved." [This should give some superficial hardness]
(

http://www.bmfinishers.com/stainless_steel.html)

Let us know what you end up doing and how it works out.

 

[metman]

kenvlach,
Since many of us do not have access to the spec's you referenced, can you please tell us what the heat treatment is for hardening electroless nickel plated on 18/8 SS? Are you sure about the numbers? That equates to 65HRC correct? Also a little theory about the metallurgy of this hardening process would be welcome. Does this technique stand up to induced Hertzian stresses or would this be applicable primarily for sliding friction. Jesus is THE life,
Leonard

 

[kenvlach]

Everything you wanted to know about Electroless Nickel (well, almost):
ASTM specs. are available from ASTM (

http://www.astm.org)

for about $35 each, AMS specs. are from SAE (

http://www.sae.org)

for $59, MIL specs. are available on the Defence Department's ASSIST site (

http://assist2.daps.dla.mil/quicksearch/)

for free, but certain specs. are 'Controlled Distribution Document,' including MIL-DTL-26074F & MIL-C-26074E.

Here is a good, brief introduction to Electroless Nickel:

http://www.pfonline.com/articles/pfd0024.html

The Metal Finishing Guidebook explains the electroless reaction, gives solution formulas, deposit properties, etc.:

http://www.metalfinishing.com/WZ/MetalFinishing/guidebooks/MetalGB_PlatingProcedures/000012/show/

"EN Specifications: Working Tools for Industry" is at

http://www.pfonline.com/articles/019707.html

This article presents a portion of ASTM B733 relating to heat treatment of EN-plated parts:
Post Treatment. Classes of post heat treatment:
Class 1: As deposited
Class 2: 850 Knoop hardness minimum
Class 3: Heat treat at 180 to 200C for two to four hours to improve adhesion and provide hydrogen embrittlement relief.
Class 4: Heat treat at 120 to 130C for at least one hour to improve adhesion on heat-treatable aluminum alloys and carburized steel
Class 5: Heat treat at 140 to 150C for at least one hour to improve adhesion for non-age-hardened aluminum and beryllium alloys

From MIL-C-26074,
The required hardness for Class 2 can be achieved by heating according to any of the following schedules:

Temperature (F) Hours
500 4 or more
550 2 or more
650 1 to 1.5
750 0.5 to 1

[I prefer 500 or 550 F; higher temps cause tarnish in air.]

"Physical Properties of Electroless Nickel -Phosphorous Deposits" including hardness & Taber wear results, is at:

http://www.macindustrialproducts.com/properties.html

Re metallurgy: The as-deposited EN is an amorphous, metallic alloy of Ni and between 1 & 13% Phosphorus (depending upon plating solution), most are 5-8.5% P.
The hardening mechanisms are the formation of NiP precipitates within the solid solution-hardened crystallizing coating matrix. See below:

"Deposit Stucture
Hypophosphite reduced electroless nickel is one of the very few metallic glasses used as an engineering material. Depending upon the bath formulation, deposits may contain from 1 to 13% phosphorus dissolved in nickel. The structure of these coatings depends upon their composition. Deposits containing more than 8.5% phosphorus have no crystalline structure or separate phases and are normally amorphous to x-rays. Deposits from 5 – 8.5% phosphorus contain different phases of nickel and are partly crystalline. Those deposits below 5% in phosphorus content and crystalline are typically laminar in structure...

For "Mechanical Properties" i.e., see

http://www.macindustrialproducts.com/mechanical.html

selected portions:
"Hardness & Wear Resistance
One of the most important properties for many applications is hardness. As deposited, The micro-hardness of electroless nickel coatings is about 500 to 700 HK100. That is approximately equal to 45 to 58 HRC and equivalent to many hardened alloy steels. Heat treatment causes these alloys to age harden and can produce hardness values as high as 1100 HK100. That is equal to most commercial hard chromium coatings...

"Hardness & Wear Resistance
Electroless nickel coatings have excellent resistance to wear and abrasion, both in the as-deposited and hardened conditions. Laboratory tests have shown fully hardened coatings to have wear resistance equal to hard chromium under both dry and lubricated conditions...

"Adhesion
The adhesion of electroless nickel coatings to most metals is excellent. The initial replacement reaction, which occurs with catalytic metals, together with the associated ability of the baths to remove submicroscopic soils, allows the deposit to establish metallic as well as mechanical bonds with the substrate. The bond strength of MacDermid EN coatings to properly cleaned steel has been found to be 400 MPa (60 kpsi) or more. The adhesion to aluminum and aluminum alloys is less, but usually exceeds 300 MPa (40 kpsi).
With non-catalytic or passive metals, such as stainless steel, an initial replacement reaction does not occur and adhesion is reduced. With proper pretreatment and activation, however, the bond strength of the coating normally is at least 140 MPa (20 kpsi). The adhesion to copper alloys is usually between 300 and 350 MPa (40 and 50 kpsi).
With metals such as aluminum it is common practice to bake parts after plating for 1 to 4 hours at 130 to 200 C (270 to 400 F) to increase the adhesion of the coating. These treatments stress relieve the part and the deposit and provide a very minor amount of codiffusion between the coating and substrate...

With regard to cyclic stress & sliding friction:
EN is very good w.r.t. sliding friction, there is lots of data for both dry & lubricated cases.
Re cyclic stresses, EN coatings should be very good in general up to the adhesion limit since EN is very strong and has little deposit stress, especially after heat treatment. ("The ultimate tensile strength of most coatings exceeds 700 MPa (100kpsi). That is equal to many hardened steels and allows the coating to withstand a considerable amount of abuse without damage.&quot
However, there could be a problem in high strain situations, e.g., a thick EN deposit upon sheet metal which is severely bent, since the EN has limited ductility.

I should mention that the EN adhesion test for certifying to the various specs. is very severe: "4.5.2 Adhesion test. The test specimen shall be bent 180o over a mandrel [of] diameter 4 times the thickness (0.4 inch minimum) of the specimen..."[examine under magnification, try detaching with a sharp blade, then continue flexing 180 degrees until the substrate fails. the EN is allowed to fracture, but not to separate from the substrate, even with further use of a sharp blade]

 

[TEV]

Kolsterizing is a process for case hardening austenitic stainless steels that will result in a case depth of 1 to 1.5 thousanths of an inch and a hardness in the range of 50 to 60 Rc. Since it is an infusion process, there is no issue with adhesion. The corrosion resistance of the treated product is at least equivalent to the solution annealed and quenched product in virtually all corrosive exposures. It is considerably superior in stress corrosion resistance due to a high degree of residual compressive stress.

 

[paul423]

TEV correctly states many facts about the Kolsterising process. However, the surface hardness is equivalent to 70 to 74 HRc.