Hardfacing nickel based powders to AISI 431 Stainless Steel

[attin]

Greeting all,

We are producing nickel and iron powders for hardfacing process. our customer complaint about us why they arent using any hardfacing alloys to coating AISI 431 Stainlees Steel. Hardfacing materials are seperating after the application from this material. They are using spray and fuse methods. Already now, thanks for your helping. Good works...

[Gordon]

Hi attin

to the Surface Engineering Forum.

AISI 431 is a martensitic stainless steel and could be causing problems. How is your customer cooling the parts after fusing? The self fluxing alloy type will also have an effect as the harder/less ductile coatings will also make the situation worse.

[attin]

Thanks for reply Gordon,

They are cooling in room temperature.I mean, when they completely coating powder on the AISI 431, it is automatically cooling in the process room. when we looked on the material surface, coating gradually leave on the material surface.Powder particle size range about 44-100 micron, hardness about 38-43 HRC. Good work..

[Gordon]

Hi attin

AISI 431 needs to be cooled from fusing temperature extremely slowly to avoid the martensitic transformation (hardening) along with resultant expansion that occurs at low temperatures ~ 100-200 C. Allowing the substrate to transform in this manner causes great stress on the relatively cold and non-ductile coating.

AISI 431 would be best slow cooled in furnace, ideally with a hold for a few hours or more at around 650 C to ensure a gradual transformation to softer/less stressful non-martensitic phases.

Can your customer change the part material from AISI 431 to a steel more suited to accepting a fused coating? The original properties of AISI 431 will certainly change anyway, due to the fusing process.

[attin]

Hi Gordon,

Thanks for your helping, our customer hadnt been knowing for spray and fuse methods and coating which material. they have listened advice to anybody and they have bought charged a proportion AISI 431. I mean, they want to finish AISI material. Anyway,they are using Spherical pig iron at the base metal.

Gentleman Gordon, i have new question about hardness. owing to you, i will solve this problem. I sent you private email about hardness. To summarize, what is the different macro-micro hardness of powders. I mean, which method do we use?also, healthy?

yours respectfully,

Good Works,
Bahattin

[Gordon]

Hi Bahattin

If we are only taking about a powder particle, then really the only method would be micro-hardness test, simple because of size and the fact that we can not physically use the other higher force methods. If confusion is caused by people or literature quoting say 60 HRC for a particle ~100 micrometre, then this is very bad practice. To do this test is physically impossible and will be a rough conversion from a micro-hardness test. This should only be done purely for visualising hardness on a more familiar scale and should state or have reference to original hardness, test method and conversion chart used. The converted hardness value should never be used as a specification or indeed used to validate a specification/standard. Conversion between methods and different scales is not universally accurate and should only be used as a rough guide.

If we are taking about the hardness of the fused coating i.e., the finished product, (which is probably more important) then a larger range of test methods could be employed. Typically Vickers, Rockwell and micro-hardness. Rockwell methods using HRC, HRA, and superficial HR 45N, HR30N and HR15N scales (for hard fused coatings) are convenient, easy and quick. Scale selection is usually based on thickness of coating. Make sure there is sufficient coating thickness to support that specific scale i.e. if testing coating at around 60HRC with HRC you will need in excess of 0.050" of coating thickness to be sure of a true result, while HR 15N at 90 (rough equiv or 60 HRC) will only require thickness in excess of 0.010".
Again it is important that the same test method and scale/force is used as quoted from the specification or standard that you are working with. Using converted hardness values between different methods/scales/loads should be treated as approximate guide and not as proof of conformance.

Hope that helps.

More info https://www.gordonengland.co.uk/hardness/ and associated links

[attin]

thank you very much Mr. Gordon,

I have little experience about hardfacing powders, in the beginning, we were producing brazing and sintering parts powders. Hardfacing is a complex. Hardness is the most significant thing for the hardfacing. But such as Cr-B-Si-P elements what are they rolling in the hardfacing powders. I know, CrB, NiSi hardness compound or CrC, but i dont control this element which ratio influence hardness.I guess, you are well informed hardfacing. Can you help me about hardfacing powders charesteristic feature? .i know, i have many question. i 'm researching but i dont find many source about hardfacing. Thanks for advance, Mr. Gordon. Good Works..

[Gordon]

Hi Bahattin

Well, I'm no expert on NiCrBSi chemical recipes and manufacture. Boron and silicon additions, though they can have some effects regarding hardness, they are primarily added as fluxing agents. They reduce the melting point of the alloy, they scavenge oxygen and I know getting the balance of these constituents right is important to the fusing properties of the alloy.

Carbon probably has the biggest influence on hardness.

[attin]

Hi,Gordon

I want to ask you, again question. how do we choose hardness for hardfacing process?I mean, for example, for glass technology die, how do we choose hardfacing methods?The things what we should have to pay attention on.Thanks for advance, Mr.Gordon. Good Works..

[Gordon]

Hi attin

I want to ask you, again question. how do we choose hardness for hardfacing process?I mean, for example, for glass technology die, how do we choose hardfacing methods?The things what we should have to pay attention on.Thanks for advance, Mr.Gordon. Good Works..

How long is a piece of string? Sorry, question not easily or quickly answered. Even dealing with a specific application with all the details of the required coating function is not always such an easy coating selection task. I could say " higher hardness = higher wear resistance" but this is far too simplistic and not always true. Hardness is just among one of many properties to consider.

I would take a close look at some of the leading powder suppliers standard/off the shelf self-fluxing alloy powder range, which generally cover most general use. Some of these companies however, will supply special products which are tailored specifically to a customers (glass industry certainly one) particular application and needs.

[djewell]

Would your customer consider a different application method besides spray and fuse? This can be a troublesome application method because of the way the coating changes during fusing. If they were to use AC-HVAF or HVOF for example, they could spray a hard facing coating and be done. They would save a lot of time. Also, they could manipulate their parameters to achieve compressive stress in the coating so when the part cools, the coating relaxes to a neutral stress state, and delamination will not occur.