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Key Points
HFMI Peening is the most effective method to increase fatigue life and prevent cracking
HFMI (High Frequency Mechanical Impact) refers to peening at 100Hz (6000bpm) or more
Specialised tooling is required to achieve the required results - needle guns and air chisels are not adequate in terms of impact energy and frequency
Whether ultrasonic or pneumatic, all specialised HFMI peening tools are equally effective.
Weld-Hit is the most affordable specialised HFMI equipment available. Any welder can use it once competency training is completed.
If you want to improve the fatigue life of welds, there are a few options you could consider.
From best to worst in terms of their effectiveness, they are
Peening
TIG
Smooth blend
Toe burr
Grinding disc (along toe)
Peening is the best and most effective method to improve fatigue life and prevent fatigue cracking. To put it simply, peening creates localised deformation to change the surface profile and induce surface compressive residual stresses. This provides up to a 10x improvement in fatigue life, or may prevent some cracking issues altogether.
Peening may be broadly classified in 3 groups
Inter-run peening (to remove residual stresses formed during welding)
surface peening (eg or a weld cap or crack prone area of a casting)
weld toe peening (particularly of fillet welds)
Inter-run peening is an essential strategy when doing localised repairs of cracking or defects in order to prevent the very high tensile residual stresses that are normally generated. These residual stresses reduce fatigue life and accelerate crack growth. Quite a lot of deformation of the weld metal is required to sufficiently expand it to counteract the shrinkage on cooling. For this reason, a needle gun just isn’t sufficient.
There are many ways to do surface peening (shot peening, needle gun, ball-pein hammer, air impact tool etc), with a common goal of achieving sufficient coverage and inducing enough deformation for the intended effect. Needle guns can have limited effect for certain applications, but do not produce an optimum outcome compared with the more intense peening effect produced by specialised peening tools.
Weld toe peening is done by using a peening tool with a specially shaped impact bit to produce a localised groove at the toe of the weld. Weld toe peening is also called High Frequency Mechanical Impact (HFMI) because of the specialised tooling required to do it effectively. High frequency in this context is defined as ≥100Hz, meaning the impact tool is hitting the steel at 6000bpm or more. The high frequency impact is required to produce a smooth, uniform groove.
There are a number of different options available for peening equipment in terms of how they are driven, and the impact frequency. These are compared in the table below
Needle guns just don’t induce enough deformation to be an effective peening option - but are better than nothing for surface peening.
Although an air-chisel can have limited application, it is not really a viable or suitable alternative. The longer stroke length and insufficient impact rate (BPM) makes it hard to control and makes the work take much longer to complete. Furthermore, an important safety consideration is that suitably shaped impact bits just aren’t readily available for the required applications, requiring chisel tools and points to be ground back to change the shape of the bit. This can introduce safety hazards if the bits then break or fragment, as well as make it impossible to control the quality of the peening. Also, high hand vibration levels are dangerous with prolonged use.
So for a HFMI Peening tool, this leaves us with the choice between Ultrasonic and Pneumatic. Both do the same thing - produce controlled surface deformation via high frequency mechanical impact. Both have specialised impact tools for various peening applications. The main difference is in how the impact energy is created. Weld-Hit is a pneumatic tool which uses an internal piston that drives the impact energy, except that it creates a high energy impact at 6000bpm compared to the typically 2000 to 3500bpm of an air chisel or needle gun.
In comparison, an ultrasonic peening tool such as uses an ultrasonic exciter (actually, it is called a magnetostrictive transducer) instead of a pneumatic system to generate vibrations that travel through a wave guide, which drives the impact bit to hit the surface at between 100-200Hz. It is important to note that although the frequency of the ultrasonic exciter is often quoted in sales literature (eg 25kHz), this is NOT the rate at which the tool is impacting the surface. This is just the frequency of the exciter unit. The ultrasonic frequency is not transmitted into the steel.
In short - whether it is ultrasonic or pneumatic HFMI, the actual effect on the steel is the same (provided it is done correctly). They are equally effective.
So which one is right for you? First of all it depends on whether you want to buy the peening equipment, or pay someone to do it for you. If you are paying someone else to do the work, then the equipment used is less relevant - providing the technicians are trained and competent. But if you are needing to do a lot of peening work on a routine or ongoing basis (eg inter-run peening of crack repairs or toe peening of large fabrications) then selecting the right HMFI equipment is essential.
The following table summarises the major brands of HFMI equipment in terms of whether they are ultrasonically driven or pneumatic, and comparative price
The following table provides a simple feature comparison between Ultrasonic Peening and Weld-Hit.
Weld-Hit democratises HFMI peening by providing an affordable, versatile, easy-to-use and highly effective peening tool for fabrication and maintenance welding. Weld-Hit goes beyond just doing HFMI peening of weld toes and surfaces - it is also highly effective for inter-run peening. Weld-Hit is unique in that it is the perfect tool for routine maintenance welding, and comes with both dedicated impact bits for inter-run peening, including a long impact bit to be able to access deep repairs.
Full competency training and assessment is provided, with formal certificates issued via the MWC Portal
For more information about Weld-Hit and all purchase enquiries in Australia, please visit https://www.wqms.com.au/weld-hit.
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