Waterjetting 28c - using steel as an abrasive

One of the considerable disadvantages in using garnet and other similar minerals as a cutting agent in abrasive waterjet cutting is that the particles fragment during the mixing process, and when they hit the target. As a result (as noted in the last post) less than 50% and often much less than that can be usefully recycled. The distinction in adding the word usefully relates to the need to remove the finer particles from the recycled stock since that does not cut very well.

But what if we used an abrasive that is not degraded in the mixing process, and further one that can be more easily separated from the cuttings and spent water? The candidate is steel, which can be formed into small particles that do not degrade in size as they move through the mixing chamber, and generally hold shape even after they have hit the target. Steel also has the advantage that it can be magnetically removed from the jet stream as the flow is collected, and with no significant degradation in size it can then be readily recycled. In cases where we have monitored the recyclability of steel shot, we were able to re-use it more than fourteen times without seeing any degradation in performance. Re-using it this many times more than offsets the increased price of the original material, and will, in a short time, also pay for the relatively low costs of a magnetic separator.

Unfortunately it is not quite that simple a choice. There are a number of other considerations, which must be addressed to make the system work effectively, some of which may make the process too expensive. Three of the areas that need to be addressed form the subject of this post.

The first comes about as a result of the shape of the particles, and their retained mass and velocity on leaving the focusing tube. More than most other abrasives steel retains some elasticity during the cutting to the point that where the cutting and rebounding streams are not carefully confined, the particles can escape upwards into the cutting room. Once in the air they move at high speed, and bounce around the room, so that they can reach unanticipated places and can also be a hazard to folk doing the work.

Figure 1. Slot depths cut into granite by steel shot (left) and garnet (right)

The second problem relates to the cutting effectiveness. When cutting a brittle material the steel shot has a number of advantages, since the energy on impact is focused in the very small volume of the sphere in contact with the target. This improves the ability of the shot to generate and grow cracks in the impact zone and thereby improves the performance of the cut, over that of the mineral abrasives.

Figure 2. Relative performance of steel over garnet and sand in cutting dolomite, under otherwise similar conditions

However, when cutting ductile materials, such as metal, steel shot is not a good tool, since the focusing of the force means that the shot may get buried or just rebound from the target, without the tearing and plowing action that comes with the use of a more regular abrasive. One way to overcome the problem is to switch from a steel shot to steel grit, which is also available. The relative benefit can be illustrated by using the change from using glass spheres to using them after they have been broken into sharp fragments.

Figure 3. Effect of change in particle shape when using glass particles in cutting ductile composite material (after Faber and Oweinah).

This, by itself may not be a complete answer, since the process of making the grit makes it a little more vulnerable to abrasion and wear during the cutting process, but we have seen that it is possible to recycle most of the abrasive a number of times. However, because of the change in shape, it becomes a little more difficult to feed the abrasive into the cutting stream, and there have been occasions where the grit has bound up in the feed tube. This has, therefore, to be sized and the flow path designed, to ensure that this doesn’t happen.

Figure 4. Cuts made into tool steel using steel shot (left) and garnet (right)

The other change is to use a harder steel than normal. And here please note that there is a difference between the hardness of the steel and its toughness. As American Cutting Edge notes:

Hardness vs. Toughness: Generally as hardness increases, toughness decreases. Toughness is desirable when blades are heavily impacted, hardness when a blade is exposed to corrosive or abrasive materials.

Hardness is related to the amount of carbon in steel. Often the lower the carbon, the higher the toughness. Also, some steels do not perform at lower hardness as they were designed for use at higher hardness. . . . . . . . Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. The Rockwell or Vickers hardness scales are most commonly used in the industrial blade industry.

Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.

In general where the grit is being used to cut into other metals (which can include steels) the hardness of the cutting abrasive should be considerably higher than that of the target material.

Which comes to the third consideration, which is that steel abrasive can rust, and therefore, immediately after it has been recovered and washed, it should be effectively dried. This has proved to be more difficult to manage than originally anticipated, since, particularly where the particles are then stored for some time before re-use, any moisture present can create enough rust to “glue” the particles together. Which renders them effectively useless for further recycling and additional use.

So there are considerable pitfalls that can arise in making use of steel as a cutting abrasive, but where the jobs exist where it does effectively cut significantly better than the alternative (say in rock-cutting applications) and where the cutting zone can be shielded, and the particles rapidly recovered, dried and stored for relatively rapid recycling at an economic price, then it can be a productive way of reducing cost, while improving throughput. (And lest you think this is a new idea Gulf Oil did extensive work on abrasive jet drilling of oilwells starting in the mid-sixties, with some favorable results, but that is another story).

Waterjetting 18c - Abrasive choices

Picking the right abrasive can make a big difference in the profit that a waterjetting operation makes. But the question, of course, is which abrasive is the best? And, as I have done in the past, I am going to hedge a little in my answer. The reason for this is that there are different factors that control the price of the abrasive – how far has it to be transported, how it was prepared and what it is made of – for example. And while some abrasives generally cut better than others, if the unit is only going to be cutting a narrow range of material, then the abrasive that is best for cutting a wide range of materials (garnet) may not necessarily be the best choice in that particular case.

Figure 1. Different types and sizes of abrasive particles.

And further, just to make life a little more complicated, not all garnet abrasive (or other types of abrasive product) are created to give equal performance. As I have mentioned in a previous post the cutting performance of the abrasive can be rapidly reduced as the particle size falls below 100 micron. Thus, if the particles have not been well graded, so that there is a significant amount of abrasive below this size (even though the vendor tells you that it is a 250 micron size) then the cutting performance will not be as good as it would be if the mix contained no fine fraction that far below the stated particle size.

Figure 2. The effect of particle size on cutting performance

On the other hand if particle sizes are too large, then for a given abrasive feed rate there will be a smaller number of particles hitting the surface, and this can also reduce cutting performance if carried too far.

Figure 3. The effect of increasing particle size on the cutting of cast iron at a constant abrasive feed rate (0.88 lb/min) (after Hashish*)

There are other factors that has also to be considered in selecting the best abrasive, and, while I am going to discuss this below, in most cases it is going to be something that you will have to test in your own shop, comparing the results that you get with the cost of the abrasive (often worked out in dollars per square foot of cut, or similar unit) to decide which gives you – in your particular circumstance – the best performance.

But the testing of different abrasives can be reduced to a manageable range of tests (and we prefer the triangle test that I have described in an earlier piece) if some basic thoughts are born in mind.

Decide what it is that you will be cutting – is it mainly going to be a metal that is going to respond in a ductile way when cut by the abrasive. In which case the abrasive should have enough sharp edges to cut into the material, and then to plow up some of the surface so that as more particles hit the surface, pieces are broken off.

Figure 4. Mechanisms for cutting into a metal or other ductile material.

On the other hand, if the material that you are going to cut is a brittle one, say for example rock or glass, then the material is going to be removed by crack growth. Here relatively spherical particles can be more effective because the energy of the particle is concentrated at the point of impact, and more easily causes cracks to grow. On the other hand relatively flat particles with multiple impact points reduce the pressure under any one and reduce the effectiveness of the particles in getting the cracks to grow as quickly and as long as possible.

Thus, for example, we can compare the effect of using the same amount of steel shot (round) and garnet in cutting granite (a brittle material) and tool steel.

Figure 5. Cuts in granite and tool steel using the same abrasive feed rate, but the cut on the right is with garnet and that on the left is with steel shot. Note that the steel cuts the granite to a deeper distance within the cut, while in the tool steel it bounces off without leaving much impression. (However harder steel grit on softer steels can be an effective choice).

The difference that particle shape makes in cutting ductile materials can be shown when the same AFR is used but in one case the abrasive is broken glass particles and in the other it is glass beads.

Figure 6. Effect of particle shape (broken glass and glass beads) in cutting composite material at the same abrasive feed rate (after Faber***)

Further some particles, for example steel shot or grit, can be recycled through the system a number of times without much degradation, so that if they can be simply collected below the cut, they can be profitably reclaimed, providing that the particles below 100 micron are separated out of the flow after each cycle.

Figure 6. Effect of recycling abrasive through a slurry abrasive jet system on depth of cut achieved (after Kiyoshige et al**)

Others however are more friable, this is perhaps more true of alluvial garnet, which often has a much higher density of internal cracks than mined garnet, and so is more liable to fracture either in the mixing chamber or on first contact with the target, so that the amount that remains above 100 microns is substantially smaller than that with the mined alternative.

Yet cost is a factor, and so it is best, for your material to test the different alternatives available, before making a final decision.

* Hashish, M., "Abrasive Jets," Section 4, in Fluid Jet Technology, Fundamentals and Applications, Waterjet Technology Association, St. Louis, MO, 1991.
**Kiyoshige, M., Matsamura ,H., Ikemoto, Y., Okada, T., "A Study of Abrasive Waterjet Cutting using Slurried Abrasives," paper B2, 9th International Symposium on Jet Cutting Technology, Sendai, Japan, October, 1988, pp. 61 - 73.
*** Faber, K., Oweinah, H., "Influence of Process Parameters on Blasting Performance with the Abrasive Jet," paper 25, 10th International Symposium on Jet Cutting Technology, Amsterdam, October, 1990, pp. 365 - 384.