Even when you control the metal sheets with clamps, the welding operation creates a number of defects in the welded joints. These welding defects can reduce the static resistance of the joint and often have a major influence on its fatigue strength over time.
The shot peening treatment of the welded joint helps improve its fatigue strength and reduces the geometrical stress concentration. Peening can be applied to both shallow surface cracks and fillet welds that have not cracked to improve the fatigue resistance of the weld.
Weld Distortions
Before we dive into peening lets talk a bit about welding distortions. Welding distortion is caused by the expansion and contraction of the weld metal. This occurs when the base metals are heated during the welding process. There are multiple reasons why this happens.
When you perform all the welding on one side of a metal joint, it will cause much more distortions than if the welds are alternated from one side to the other.
During the heating and cooling cycle that is common in welding, many factors affect the expansion and shrinkage of the metal and cause distortions to appear. These factors include the physical and mechanical properties of the metal that changes as heat is applied.
As the temperature of the area under welding increases due to the arc application, the elasticity of the metal and conducting capacity of the steel plate decreases. These changes in metal properties affect heat flow and uniformity of heat distribution across the weld area.
Peening To Reduce Weld Distortions
Peening is an effective method used to balance the shrinkage of a weld puddle as it cools down. Generally, peening the weld bead thins it out by stretching it over the surface. This helps reduce the stresses induced by the contraction of the metal as it gets cooler.
Peening is considered a cold working process. The surface of the weld component is deliberately deformed, with the help of an air hammer.
During the peening process, the surface layer of the weld may appear to expand laterally. However, it is prevented from actually expanding by the elastic nature of the bulk material under the surface. This leads to the formation of compressive but beneficial internal stresses in the surface layer that gets a corresponding balance by internal tensile stresses elsewhere in the weld.
The process of preening creates a layer of compressed surface material that resists the development and propagation of cracks. It also improves the weld’s resistance to fatigue failure, stress corrosion, strength fatigue, and cavity erosion.
Types of Peening Techniques
There are four main types of peening techniques used in welding.
Shot Peening
This is the most common method for weld peening. It involves hammering of the metal surface by a spherical shot from an air gun. The shot also includes tiny, rounded particles of various components in different mesh sizes.
In most cases, a high-velocity stream of air is used to propel the shot onto the surface. Either manual or mechanized peening systems can be used, depending on the particular welding project and area under treatment.
Roto (flail) Peening
In roto peening, the surface area is compressed with the help of multiple flaps that are shot-coated and extended sideways to form a spindle. The flaps contain shot (metal or tungsten-based carbide) that are embedded into a fiber of wire mesh.
The tool is chucked into a drilling tool. This drill is rotated at a speed of about 3000rpm. The flaps are brought into contact with the weld surface to create the peening effect. The energy applied to the weld is determined by the rotational speed of the spindle.
Needle Peening
The needle peening process uses a de-scaling gun with multiple needles that have a radial shaped needle head. The needles are usually made from a hardened tool steel metal.
During this peening operation, the weld surface under treatment receives multiple and repeated impacts from the needle gun, starting from the weld toe to the bead in a criss-cross motion.
Hammer Peening
Hammer peening is similar to needle peening with the only difference that it uses a single metal rod instead of a cluster of needles for peening. The hammer is applied in the same manner as the needles during the needle peening process.
The basic method relies on using the ball end of the hammer. This peening technique is often used as a means of applying stress relief between adjacent weld when making welds that will be subjected to high stress and structural fatigue.
Areas to Focus for Peening
When it comes to peening there are three main areas of concern where the process can be useful; cracking of the weld or base metal to reduce stress corrosion, obscuring or masking of weld distortions and work hardening the superficial layer an area.
- Basic shot peening is used to apply compressive stress that reduces cracking due to stress corrosion or strength fatigue over time.
- Corrections can be imposed by stretching or compressing a localized area. This helps achieve perfect flatness for the weld surface when it has been distorted by heat, complex structural shapes or machining of the surface.
- Peening can be used to harden the work surface to form skins, wing flaps, access panels and/or other sections of fuselage.
Where is Shot Peening Applied?
Shot peening is generally used in industries where metal structures are exposed to a lot of stress. This includes rocketry, aircraft, boats and auto industries. Vehicle frames need to be repaired after welding to relieve any stress in the structure that may be present due to the machining or heating process.
Peening replaces these metal stresses with beneficial bonds. The residual stress that is present in metal bonds is produced due to the transfer of the bonding energy that protects it from wear and tear with time.
The preening process is used to achieve three goals. It must be:
- accurate
- reproducible
- controlled
Aerospace, naval production and auto sectors are critical for peening. Welders must always follow high standards and carry out peening to enhance the mechanical properties of structures that are going to be under considerable stress during use
Control the Peening Process for a Better Finish
The use of needle scalers, chipping guns and slag hammers to smooth out the weld surface is not peening when you are trying to remove slag. Once the slag has been removed, using these tools on the metal joint surface by overworking an area can even cause distortions.
The value of the residual stress for your joints depends on several variables including the parameters of the peening process and the hardness of the component material.
It is important to achieve the correct intensity of stress as the preening process is often used to improve the performance of components that are critical to safety. Repeating the process with similar variables should lead to the same level of structural integrity.
The Almen Strip
The Almen Strip testing procedure can help take the guesswork out of the preening process. It is used to predict with fair accuracy the energy that is being applied by each shot.
The Almen strip measures the strict tolerances of hardness and flatness. The strip is peened on one side by a shot peening device. The induced compressive stress on the strip results in the strip becoming bowed or curved.
The degree of curve is calculated to be comparable to the energy imparted by the shot. The ‘Almen Gauge’ is used for this purpose.
The height of the Almen strip arc varies according to both the velocity and mass of the shot i.e. the amount of energy given out by the stream of the shot and impact that is absorbed by the strip.