A sound way to strengthen steel

Tom Shelley describes a technique for enhancing fatigue strength using
ultrasound that has a surprising number of other uses

May 2002 Feature story: A sound way to strengthen steel

Ultrasonic finishing is being used to greatly enhance the fatigue strength of steel and other metal alloy components in a fraction of the time required by alternative techniques and at a much lower cost.
It can be incorporated into a machining process as a separate step, without needing to remove the workpiece, and is already in commercial use in Russia where experience of the process and its capabilities is beginning to build.
Ultrasonic finishing was primarily developed for treating crankshafts and spherical and conical surfaces. However, ultrasonics also appears to work well as a means of eliminating lime scale build-up in hot water and steam systems running on hard water. It also has a host of other uses.
The technology is the result of a lifetime's research work by Professor Yuri Holopov. This has now spun off from St Petersburg State Technical University into the North West Centre of Ultrasonic Technology or INLAB.
The underlying principle is the same as that used in shot peening and vibratory finishing (Eureka, March 2002) and super precession polishing (Eureka, April 2002), which is to put the surface layers of the material into compression to produce a smoothed, polished finish. Both effects are known to be effective, because they remove defects which may act as fatigue crack initiation sites, and close up any that are open.
The underlying technique is to put a tool with an ultrasonically driven hard steel tip into the tool post of a lathe. It does its work as the last stage of a set of machining operations - without requiring removal of the workpiece from the machine.
Ultrasonic finishing can be undertaken in similar timescales as finish machining, Professor Holopov mentioning feed rates of 0.05 to 0.2mm per revolution for crankshaft journal polishing at machining speeds of 500 to 600rpm. Residual tensile stresses of 300 to 400N/mm2 are reversed and turned into compressive stresses of 240 to 700N/mm2. The depth effect is 50 to 500 microns, which is slightly higher than for shot peening. Holopov claims general fatigue strength increases of around 50%, depending on circumstance and fine machined finishes of 1.6 to 3.2µm are turned into mirror finishes of 0.2 to 0.4µm. The fatigue strength enhancement is less than that claimed for shot peening under NC control plus vibratory finishing under best circumstances, but seems realistic.
If engine crankshafts could be made thinner and lighter, even by a small amount, it has an immediate, fairly dramatic effect on engine acceleration and fuel consumption. Research by most motor manufacturers on improved crankshafts seems to be mainly focussed on using exotic materials, such as metal matrix composites. Shot peening and vibratory finishing has been shown to bring such dramatic benefits to conventional steel gears and gearboxes, but only at motorsport-type prices. Ultrasonic finishing looks to be a lot cheaper than either technique.
The tools are driven using magnetostrictive technology and deliver power outputs of 400 to 600W. They are made in two parts: an acoustic head in contact with the workpiece, which weighs 4 to 6kg; and a separate electronic converter, which turns the normal 220V 50Hz mains supply into 22kHz. The latter weighs 7.5kg.
About 100 sets are currently in service in Russian factories with more on order and a large number of serious enquiries under discussion. The technique has been found to give benefits to mild, construction, tool, cast, alloy and bearing steels; gray cast and wrought iron; and copper, brasses, bronzes and aluminium alloys. It has been successfully applied to shafts, faces, conical and spherical surfaces (external and internal), grooved channels, fillets and the insides of blind holes
Selling price for a complete set is currently $8,000, down from $15,000 a short while ago. The company generally seems to be cutting prices as it gets its production under way and the equipment comes with a two year guarantee.

Sound softened water
As well as increasing the fatigue strength of metals, Holopov finds that ultrasound is also a good way of preventing the build-up of scale on the insides of hot water pipes and boiler tubes.
Scale build-up is a hugely expensive problem. A 1mm scale layer, for example, on the inside of a typical heat exchanger can result in an increase in fuel consumption of 2 to 2.5% and a 5mm scale layer requires a fuel increase of 8 to 10%. Overheating and corrosion under the scale layer can also substantially reduce tube life.
The most common way of softening water is to use chemicals to precipitate out lime and other contaminants; ion exchange is another. Both procedures incur significant cost. There have, however, been persistent reports that various other techniques can cause the precipitation of calcium salts as a fine suspension of apatite crystals, instead of calcite coatings - eliminating harmful effects at much lower cost. We are already aware of the use of permanent magnets and radio frequency magnetic fields but Holopov and his colleagues recommend low levels of ultrasound.
Research into this method apparently began in the USSR in the 1930s, and started to see service in the 1950s and 60s. However, it is only now that the cost of the equipment has dropped to a point where the method is economically attractive.
As well as initiating precipitation within the bulk of the water, ultrasound apparently offers the additional benefit of being able to initiate fractures in - and eventual break-up of - existing scale coatings. Power consumptions in INLAB's commercially available equipment is 30, 70, or 380W, depending on application.

A plethora of potential
Other processes mentioned by the company include: reinforcing of plastic with metal; hole broaching and engraving of ceramic, glass and stone; reduction of residual stresses in welds; dispersion and de-aeration of fluids; metal finish turning, drilling, countersinking, hole reaming and threading; removal of cast ceramic from a casting hollow; and 'impregnation of friction pairs with geomodificators'.
This last is apparently a new process for incorporating naturally occurring mineral ceramic materials into the surfaces of metals, thereby reducing friction and wear. We are told that, in internal combustion engines, it can increase intervals between repairs and servicing by four to six times and reduce fuel consumption by 8 to 15%. For further details, watch this space! UTINLAB

Ultrasonic vibration may be applied to a smoothing tool in a final finishing operation to induce compressive stresses in surface layers, greatly enhancing fatigue life

Ultrasonic vibrations may also be used to prevent and remove scale in pipes carrying hot water and in a host of other applications

Tom Shelley

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