Fact: HFMI is a well-established and widely utilized technique with a strong scientific basis. It has been recognized and incorporated into international standards and guidelines by major organizations, including the International Institute of Welding (IIW) and the American Association of State Highway and Transportation Officials (AASHTO). This formal inclusion demonstrates that HFMI has moved from an emerging technology to a mature and reliable engineering solution. 

Fact: While the concept of peening is old, modern HFMI technology is a highly controlled, reliable, and user-friendly method for improving fatigue strength. Unlike conventional hammer peening or air chisels,  HFMI tools are based on impact frequencies of 100 to 200Hz with very short stroke length. This means precise process control and superior results. The other benefit of proper HFMI tools is that they come with the correct impact bit types to produce the required profile at the weld toe. The technology is now so advanced that automated and robotic solutions are used in mass production to ensure accuracy and efficiency. 

Fact: The same effects as HFMI peening cannot be achieved using a needle gun. A needle gun will not be able to create the required amount of deformation, or achieve the rounded profile at the weld toe. A needle gun cannot be used for inter-run peening to prevent distortion and residual stresses. It simply cannot create sufficient deformation to counteract weld metal shrinkage. 

However, a needle gun with worn, rounded needles can be used to create an effect similar to shot blasting on the surface of a component.

Fact: Air chisels cannot be used for HFMI peening of weld toes. HFMI requires impact rates of greater than 6000bpm (100Hz) and short stroke length to create the required deformation and maintain control. Most air chisels are 2500-3000bpm and have a long stroke length, making them impossible to control. Secondly, specialised impact bits are required to produce the profile - these are only supplied with dedicated peening tools. Chisel tools should never be ground back to a different shape - this creates significant safety risks due to tool fragmentation. 

Lastly, HFMI peening tools are designed to have low hand vibration levels compared to the very high hand vibration levels from air chisels 

Fact: Ultrasonic Impact Treatment (UIT) is a very effective peening technology, but fundamentally it does exactly the same thing as pneumatic HFMI peening tools. Peening is the process of inducing controlled deformation. UIT simply uses a different method of creating the impact energy. Claims relating to the benefit of the ultrasonic frequencies are false. The actual impact frequency of the impact tips is about 200Hz - not >20kHz like the ultrasonic frequency of the sonatrode which creates the impact energy. 

At the end of the day, UIT has some specific applications where it excels, but it is heavy, large equipment that is impractical to use in many other applications. The choice of peening tool brand or type is up to the user providing that it meets the requirements for HFMI Peening - all deliver similar results in the hands of a trained operator. 

Fact: There are a range of HFMI peening equipment available, ranging from between approximately  $5K AUD to more than $75K AUD.. The key is to choose the appropriate tool for the task. Weld-Hit is the most cost effective equipment and is a simple to use, highly portable pneumatic tool. It is ideal for maintenance work where every boilermaker can have one in their kit.  

More specialised equipment such as PITEC and Hi-Fit have additional features regarding control of the intensity, making them more suitable where quality control is required during fabrication. At the most expensive end of the spectrum are the UIT devices. These also have their specific benefits, but they are also much bulkier and not easily transportable

Fact: Peening must only be done by trained and proficient operators who have an understanding of the peening process and purpose, and are proficient in the use of the peening tool. This is just the same as how welding needs to be done by competent welders, otherwise the weld will be poor quality. 

Peening done by an untrained operator is likely to be ineffective, and may even cause major damage to the structure that will then need to be repaired.

Fact: All structural welding standards have provision for peening. Generally this is as a distortion control measure, with limitations for peening on the weld root and weld cap. 

HFMI weld toe peening is not directly recognised in structural welding standards as it is considered a post-weld fatigue improvement method (not directly welding quality related). However, HFMI peening is formally recognised by IIW and is included in design standards such as AASHTO.

Fact: This belief is based on some welding standards, without properly understanding the correct interpretation. Peening of weld caps can prevent inspection for quality requirements, so weld QC must be done before peening. 

However, peening is highly effective on weld caps, acting much like shot peening to create a layer of compressive residual stresses to counteract tensile fatigue stresses. Surface peening should always be considered for crack repairs in castings, and for prevention of cracking in high stress transition areas.

Fact: The HAZ from welding can be susceptible to fatigue failure due to high tensile residual stresses and altered grain structure. The HAZ can be significantly harder than the parent material, creating a metallurgical notch, and in quenched and tempered steels there is always a softened zone in the HAZ that is of lower strength. HFMI peening is particularly effective in remediating these issues. The peening process plastically deforms and cold-works the surface, introducing a beneficial state of deep compressive residual stresses that can reach up to 2 mm in depth. This effectively counteracts the detrimental tensile residual stresses from welding, and increases the local strength at the HAZ, turning the HAZ from a point of weakness into a point of enhanced strength. 

Fact: The effectiveness of HFMI is particularly significant with high-strength steels. The IIW recommendations for HFMI are applicable to steels with yield strengths from 235 to 960 MPa , and new guidelines are expanding the scope to even higher-strength steels up to 1300 MPa. The positive effect of HFMI on fatigue strength increases with the base material's strength. 

Fact:  HFMI peening is quick to do - whether that be for inter-run peening to control residual stresses and distortion, or weld toe peening to improve fatigue life. 

For inter-run peening, it may only take a couple of minutes to do 300mm of weld. 

HFMI weld toe peening is significantly faster than toe burr grinding and TIG toe dressing. 

The HFMI peening process is cost-effective due to its user-friendliness, portability, and high reproducibility. It allows for significant reductions in maintenance, repair, and potential downtime, offering a practical solution for industries that rely on high-stress components. 

Fact: The research confirms that HFMI is a proven and highly valuable tool for the rehabilitation and life extension of existing, in-service structures. The application of HFMI on existing structures can be performed with results comparable to those on newly treated welds. This capability makes it an ideal solution for aging infrastructure like bridges, cranes and heavy equipment that have accumulated fatigue damage over time. 

Fact: HFMI has been shown to be effective on joints with existing cracks up to 1.0-1.2 mm deep The deep compressive stress field generated by the treatment acts to "close" or "shield" the crack flanks, thereby significantly arresting or mitigating crack growth. This ability to "repair" existing damage makes it a powerful asset for proactive maintenance and life extension. 

Fact: While burr grinding and TIG dressing are effective at reducing the geometric stress concentration at the weld toe, they do not offer the full range of benefits that HFMI provides. HFMI is a mechanical treatment that delivers a synergistic combination of three key benefits: 

• weld toe geometry modification,

•  the introduction of deep compressive stresses, and 

• surface hardening through cold-working.

This is why HFMI provides superior benefits compared to burr grinding or TIG dressing.

Fact: HFMI provides a dramatic improvement. The service life of treated welds can be extended by a factor of 5 to 15 times. In some cases, HFMI has been shown to increase fatigue strength by as much as 83% in comparison to as-welded conditions.