Boronizing - How Does It Work?

[gr8vity]

I hope there's someone here that is well versed in chemistry because I need someone to explain to me how boronizing works. What I have found out via google is this:

 

Boriding (boronizing) is a thermochemical diffusion process in which hard and wear resistant boride layers are generated by diffusing boron onto the surface of material.

 

The treatment of the materials is carried out in a temperature range of 750 to 950°C. An essential element is the optimal distribution of heat in the furnace used in order to treat all parts of a batch evenly. During the boriding process, complex intermetallic compounds are created between the elements iron, boron, chromium, nickel, vanadium, etc. The resulting borides form a hard peripheral surface layer consisting of Fe2B and other compounds. Because of its crystalline structure, the boride layer is anchored exceptionally well to the base material.

 

(Source: https://bortec.de/boronizing/?lang=en)

 

But how exactly can boron change the properties of steel?

[exchemist]

I hope there's someone here that is well versed in chemistry because I need someone to explain to me how boronizing works. What I have found out via google is this:

 

Boriding (boronizing) is a thermochemical diffusion process in which hard and wear resistant boride layers are generated by diffusing boron onto the surface of material.

 

The treatment of the materials is carried out in a temperature range of 750 to 950°C. An essential element is the optimal distribution of heat in the furnace used in order to treat all parts of a batch evenly. During the boriding process, complex intermetallic compounds are created between the elements iron, boron, chromium, nickel, vanadium, etc. The resulting borides form a hard peripheral surface layer consisting of Fe2B and other compounds. Because of its crystalline structure, the boride layer is anchored exceptionally well to the base material.

 

(Source: https://bortec.de/boronizing/?lang=en)

 

But how exactly can boron change the properties of steel?

I had to look this up and will happily bow to the knowledge of anyone with metallurgical expertise.

 

For a start it is interesting that the role of carbon in steel appears to be mainly one of preventing the dislocations in crystals that arise due to shear stress from propagating: https://en.wikipedia.org/wiki/Steel

 

However boron (which is next to carbon in the Periodic Table) seems to work differently, by forming covalently bound networks in the surface: https://en.wikipedia.org/wiki/Iron_boride   Borides in general are hard, high melting point substances so it would appear this is a bit like covering the metal with a layer of a chemically embedded glass- hard, scratch-resistant etc.

 

To speculate a bit further, I recall from my chemistry at university that boron has a penchant for multi-centre, electron-deficient bonds. Doing this in a metal, with its sea of free valence electrons, might be something that adds further stability to the bonding. But I can't find a reference to support this. 

[gr8vity]

I had to look this up and will happily bow to the knowledge of anyone with metallurgical expertise.

 

For a start it is interesting that the role of carbon in steel appears to be mainly one of preventing the dislocations in crystals that arise due to shear stress from propagating: https://en.wikipedia.org/wiki/Steel

 

However boron (which is next to carbon in the Periodic Table) seems to work differently, by forming covalently bound networks in the surface: https://en.wikipedia.org/wiki/Iron_boride   Borides in general are hard, high melting point substances so it would appear this is a bit like covering the metal with a layer of a chemically embedded glass- hard, scratch-resistant etc.

 

To speculate a bit further, I recall from my chemistry at university that boron has a penchant for multi-centre, electron-deficient bonds. Doing this in a metal, with its sea of free valence electrons, might be something that adds further stability to the bonding. But I can't find a reference to support this. 

 

Oh wait, I think I actually remember something about boron's bonds. If it's true what you're saying, then I guess it makes sense that boron hardens the material... But wouldn't boron also make the material porous?

[exchemist]

Oh wait, I think I actually remember something about boron's bonds. If it's true what you're saying, then I guess it makes sense that boron hardens the material... But wouldn't boron also make the material porous?

Why would it do that? To be porous you need pores, i.e. connected voids within the material.  

[OceanBreeze]

Oh wait, I think I actually remember something about boron's bonds. If it's true what you're saying, then I guess it makes sense that boron hardens the material... But wouldn't boron also make the material porous?

 

 

No, boronizing makes the surface very smooth as well as hard. Some years ago I tried out marine propellers that were boronized and rejected them because they were too smooth. They cut through the water with too little friction to develop the desired thrust.

[exchemist]

No, boronizing makes the surface very smooth as well as hard. Some years ago I tried out marine propellers that were boronized and rejected them because they were too smooth. They cut through the water with too little friction to develop the desired thrust.

A bit off-topic but what has friction to do with thrust in a propeller? I would have thought zero friction would be optimal.  

[OceanBreeze]

A bit off-topic but what has friction to do with thrust in a propeller? I would have thought zero friction would be optimal.  

 

 

Oh no, I am not getting drawn into a discussion on complex fluid dynamics!  

 

I will say this, the boronized propellers I tested were not a good match for the engines, rpm, speed and hull on that particular ship because the low viscous friction with the water did not yield the proper amount of engine loading (did not absorb the engine torque).

 

You can think of the propeller screwing through the water as being similar to you walking on the floor. If your shoes have zero friction with the ground they will just slip and slide without traction and you cannot advance at all. If your shoes stick to the ground, you can advance slowly but with difficulty.

 

There is some optimum value of shoe friction with the type of floor that also depends on your walking speed.

 

A similar situation arises with a propeller screwing through the water, and appropriately enough the relevant factor is called propeller slip.  Zero slip might seem to be ideal, but in fact that condition cannot happen because zero slip means no water is being moved aft. To move the boat forward, a mass of water must be moved aft in accordance with conservation of momentum. 100% slip is also not desirable because it means all of the engine power is used to move the water aft, and the boat is standing still; nothing but static thrust is being produced.

 

As in the previous example of walking, there is some optimum value of propeller slippage and that is related somewhat to viscous friction as well as several other factors including propeller pitch, diameter and rpm, just to name a few.

[exchemist]

Oh no, I am not getting drawn into a discussion on complex fluid dynamics!  

 

I will say this, the boronized propellers I tested were not a good match for the engines, rpm, speed and hull on that particular ship because the low viscous friction with the water did not yield the proper amount of engine loading (did not absorb the engine torque).

 

You can think of the propeller screwing through the water as being similar to you walking on the floor. If your shoes have zero friction with the ground they will just slip and slide without traction and you cannot advance at all. If your shoes stick to the ground, you can advance slowly but with difficulty.

 

There is some optimum value of shoe friction with the type of floor that also depends on your walking speed.

 

A similar situation arises with a propeller screwing through the water, and appropriately enough the relevant factor is called propeller slip.  Zero slip might seem to be ideal, but in fact that condition cannot happen because zero slip means no water is being moved aft. To move the boat forward, a mass of water must be moved aft in accordance with conservation of momentum. 100% slip is also not desirable because it means all of the engine power is used to move the water aft, and the boat is standing still; nothing but static thrust is being produced.

 

As in the previous example of walking, there is some optimum value of propeller slippage and that is related somewhat to viscous friction as well as several other factors including propeller pitch, diameter and rpm, just to name a few.

The analogy with friction on the floor seems misplaced to me.  A propeller doesn't grip the water by mean of friction, it accelerates it aft by means of a progressively angled blade. But I suppose we should open a new thread if we want to pursue this.