maintaining shape while abrasiveley wearing 1

[Tmoose]

Any body know of tricks to create rectangular wear?

mineral crusher dryer hammer mill.
There is a row of stationary "crusher blocks" that can be adjusted inward to reduce the working clearance with the mill running.
The hammers' working edge wears to a rounded shape and performance suffers even with tightened gaps. The wear is described as a combination of erosion and abrasion. Different hammer materials are used to resist wear, but of course some materials we crush throw corrosion into the mix, making stainless type materials necessary at the expense of wear resistance.

I seem to recall some cutting tools can "wear" to create a fresh cutting edge, but suspect that may be more of a brittle fracturing process. I'm pondering a system with multiple edges behind each other, kind of like a multi-blade razor.

 

[arunmrao]

Worn out hammers always have rounded edges . You can perhaps hard face the hammers at the edge so that they delay the wear or use carbide inserts at the edges. The difficulty with the later could be they can get dislodged.

Have you considered 2 piece hammers. You can thus replace the hammer worn out area with a new one.

Stainless steel hammers I have not encountered. Is it for a pharmacy application?

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[tomwalz]

The edge wears because the material gets pushed past it abrading it. The edge can also be seen as much thinner looking at it obliquely.

There is now a carbide with a TRS over 500,000 psi. You can braze so you get zero part loss.

In your case I would think about engineering something where you have replaceable edges.

Here are some factors I use when I explain carbide “wear”. If you do something similar in your application you may find that you can make a significant improvement with a different grade of stainless steel. Example: it may be called wear but it may be part microchipping or part chemical attack. Sometimes a softer, tougher grade will wear much better than a harder grade due to edge chipping.

Theoretical considerations
1. Wear – the grains and the binder just plain wear down
2. Macrofracture – big chunks break off or the whole part breaks
3. Microfracture – edge chipping
4. Crack Initiation – How hard it is to start a crack
5. Crack propagation - how fast and how far the crack runs once started
6. Individual grains breaking
7. Individual grains pulling out.
8. Chemical leaching that will dissolve the binder and let the grains fall out.
9. Rubbing can also generate an electrical potential that will accelerate grain loss
10. Part deformation - If there is too much binder the part can deform.
11. Friction Welding between the carbide and the material being cut
12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.
13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc.
14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal
15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets hotter faster than the inside. As the outside grows rapidly with the heat the inside doesn’t grow as fast and this creates stress that tends to cause flaking (spalling) on the outside.
16. Compression / Tension Cycling - in interrupted cuts the carbide rapidly goes though this cycle. There is good evidence that most damage is done as the carbide tip leaves the cut and pressure is released. 17. Tribology – as the tip moves though the material it is an acid environment and the heat and friction of the cutting create a combination of forces.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[btrueblood]

Tom,

I thought carbides tended to be weak in shock applications like hammers, interrupted cuts in machining, etc.

Wouldn't a monolithic (solid chunk of) carbide hammer face be more likely to crack and fail from shock loads? Would a multi-chunk hammer face of multiple pieces brazed to a ductile metal backing be a bit more durable?

Dunno, thought I'd put it out there. It may be that with the right binders, that 2nd paragraph is no longer true...

 

[tomwalz]

Dear Sir,

I think you are pretty well correct depending on how you look at things.

Carbide will break while metals will deform. However we are using carbide in saw mill saws and panel plants where they encounter all sort of foreign objects. Depending on the grade of carbide a 24” blade at 10,000 rpm can be expected to cut though 12 penny nails without damage to the blade.

Carbide is lot stronger than it used to be. When I started 30 years ago a TRS of 200,000 was strong now we sell stuff with a TRS of over 500,000 for everyday use. The binders help as does the smaller grain size, spray drying and better mixing techniques. The carbide parts are stronger because there are fewer voids, binder pools, etc. and those are much smaller typically.

Carbide might be a good face material and often is. In this case I was not specifically suggesting carbide but more suggesting the use of a list, such as presented, to break ‘wear’ down into a number of contributing factors. I used carbide because that is the list I had.

It is now common to use the braze alloy as a ductile metal backing. Theoretical maximum strength is about 0.0005” (Lucas Mishap / Handy & Harman) while impact protection rises as the braze alloy layer gets thicker.

We are working on an application with tunneling bits for mining and using about 0.003” average alloy thickness.

If you are talking about thermal stress from the differences in expansion rates then maybe a 0.010” to 0.015” thick ‘sandwich’ of braze alloy / copper / braze alloy.

Depending on the size, a monolithic piece would be expected to crack more readily. The rule of thumb is one inch in any direction.

If you go to the bottom of this page:

http://www.carbideprocessors.com/braze_alloy_choosing.htm

You will see a study we did on ductile metal behind a mechanically held machining insert. It pretty nicely supports your assumption.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[arunmrao]

I shall provide yet another example of carbide tips being used. Vertical shaft impactors have inlet and exit ports. The minerals to be pulverized are in constant contact at these ports. The wear in this region is very high.Normal wear resistant materials fail. Hence carbide tips inserts in EN8 (1045 grade) steel is used.

One difference between this application and hammers is that these ports are stationary(immobile),while in the case of hammers they are rotating. I am concerned of these additional forces on the carbide tips, remaining firm in their seat.

Normally, machine a groove press fit the carbide and then braze it is the practice adopted. I have not come across a situation that the tips chipped or broke as a brittle material. But they got dislodged,perhaps due to bad seating..

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[btrueblood]

Tom,

As always, an informative post, worth a star. Glad I prompted you!

 

[tomwalz]

Dear btruelbood,

Thank you for the kind remarks.

I must confess that I felt a certain pressure to do a good job in my reply. The quality of your posts and the courtesy with which you address issues sets a high standard, indeed.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.