SHOT PEENING: HOW DOES IT IMPROVE THE FATIGUE RESISTANCE OF SPRINGS

Shot peening is a mechanical process that has a considerable impact on the durability and performance of springs. When it comes to producing fatigue-resistant components, shot peening becomes a crucial step in the manufacturing process. In this article, we will explore the role of shot peening in spring production and how it affects their strength and durability.

The topic will be approached from different perspectives:

• Mechanical effects of shot peening: description and quantification
• Shot peening machines: characteristics of shot peening machines
• Shot beads: characteristics and peculiarities
• Parameters that define shot peening: overview

How shot peening works

Shot peening is a mechanical process that involves striking a metal surface with shot beads. In the production of a spring, this process is used to increase its fatigue resistance. During the impact, the shot bead transfers or converts its kinetic energy into plastic deformation energy. The shot bead compresses the surface of the metal, leaving an indentation.

It is easy to understand that the indented area undergoes a permanent compressive stress because the material is plastically deformed. This compressive stress zone extends beneath the material’s surface to a limited depth, typically 0.1-0.2 mm (see representation below).

The depth depends on multiple factors, primarily the thickness of the material being shot-peened. If the material is thin, it is not possible to extend shot peening to a significant depth because the material loses its original shape. The objective of shot peening is to create a compressive stress while preserving the original geometry of the shot-peened object.

Returning to the depth of the effects of shot peening, these depend on the energy used to strike the surface being shot-peened. Since kinetic energy consists of two factors, mass and velocity, the depth of the compressed zone depends on the mass and velocity of the shot beads. Shot peening time must also be considered, but once the cycle time is identified, the factors that determine the compressed zone remain linked to the velocity and mass of the shot beads.

Why use shot peening?

Let’s now discuss the reasons why materials are shot peened, particularly springs. As explained in the introduction to this series, the stress in compression/tension springs is tangential to the wire section but manifests in the material as tensionThis means that the wire surfaces tend to displace relative to each other due to tension, which acts tangentially to the wire section. If the surface is not compressed, the tension stress starts from 0 MPa and increases. If, on the other hand, the surface is compressed, we start from a negative value. In fact, defining tension stresses as positive, compression stresses are negative. This means that the stress excursion increases because it starts from a negative value, while the maximum value is fixed. It’s like moving the reference system downward, i.e., the starting point for stress calculations.

The shot-peened surface can undergo an extra tension equal to the compression value in MPa. Since the most stressed area in a spring is the surface, the benefit of shot peening is evident. In the Goodman diagram, this effect is manifested by the lifting of the upper inclined line, especially for excursions that start from low stresses for the first workload (x-axis), the left zone of the diagram. Below is a comparison between the two graphs for the same non-shot-peened and shot-peened spring.

Non-shot-peened spring graph

Shot-peened spring graph

Therefore, shot peening often represents an inseparable pair for guaranteeing fatigue resistance. In springs with highly dynamic applications (such as in endothermic engine valves), shot peening is a key factor in preventing fatigue failure. In these springs, shot peening is essential, and the focus is more on how to make this process as repeatable and consistent as possible.

The challenge is to ensure that all springs are shot peened to the same level and over the entire surface, possibly even the internal surface, which is generally not easy due to the shielding action that the coils themselves have toward their internal surface. For these springs, individual shot peening machines are used.

The visible effect of shot peening is an increase in surface roughness. After shot peening, the surface appears dimpled due to the micro-indentations from the shot bead impacts.

Non-shot-peened spring

Shot-peened spring

Surface roughness is measured using a digital image processing optical microscope.

The measurement of compressive residual stress is performed using X-ray diffraction based on Bragg‘s law. This type of analysis can mainly be conducted on flat or low-curvature surfaces. This requirement, at the risk of unreliable data, penalizes verification on springs with wire diameters less than 2.5-3.0 mm.

Read also: WHY DOES A SPRING BREAK

Pallinitrici: the machines that perform shot peening

Shot peening machines, also known as “pallinatrici” in Italian, are used for shot peening springs and typically include the following components:

• A chamber for inserting the springs, which is closed after insertion;
• A system for accelerating the shot, which can be either turbine-based or compressed air-based;
• A device for directing the shot beam, which may or may not be present;
• A system for aspirating and recovering the shot; and
• A sieve for discarding worn shot.

The size of the machine depends on the mass of the elements to be shot peened and the characteristics of the shot firing system. Shot peening small springs (wire less than 1.0-1.5 mm) presents different challenges than shot peening medium to large springs. One of the primary concerns is maintaining the effectiveness of the shot impact while minimizing the kinetic component of the springs themselves. This requires maximizing the conversion of the kinetic energy of the shot into deformation energy and reducing the movement of the springs.

Shot peening machines are mainly divided into two families:

• Turbine-based systems.
• Compressed air-based systems.

This is the system used to accelerate the shot and give them the necessary kinetic energy.

Turbine-based systems, which use a centrifugal impeller to accelerate the shot and expel it to the periphery of the impeller. Turbine machines can be fixed-speed or variable-speed, with variable-speed machines using an inverter to regulate the rotation speed of the impeller and the speed of the shot. This type of machine is typically used for medium to large sized springs.

Compressed air-based systems, which are used for small springs with correspondingly small shot. In these machines, the speed regulation is achieved by adjusting the compressed air pressure, which accelerates the shot. Compressed air-based shot peening machines are generally smaller in size and equipped with versatile systems for managing the direction of the shot flow.

Peening media: characteristics and peculiarites

Shot characteristics are defined by three factors:

• Geometric shape;
• Constituent material;
• Hardness, which characterizes its durability.

Geometric shape

Shot can have a spherical or cylindrical shape, although this is an approximation as the systems used to create the shape do not guarantee precise shapes. Spherical shot is obtained by atomizing molten steel in water, followed by sieving and heat treatment to increase its hardness. Shot is classified by size (diameter), and suppliers guarantee that bags contain predominantly spherical shot with a diameter between two min-max values.

The photograph shows spherical shot with a diameter of 0.5 mm. Shot grains for shot peening are classified according to SAE AMS2431 standards.

There are also variants within these two main families, including conditioned shot. Conditioning is a treatment that rounds off the edges of shot. This treatment is particularly important for shapes that can create pronounced edges, which may have more detrimental than beneficial effects on the surface being shot peened. If the impact creates a groove, the area becomes a potential fatigue failure initiation site.

Cylindrical, conditioned shot is obtained by breaking a round wire into cylindrical pieces, which are then rounded by flattening with rotating cylinders.

Cylindrical conditioned peening

Materials

There are four main materials used to make shot: metals, ceramics, glass, and natural compounds with high hardness.

Carbon steels with a carbon content between 0.7-1.2% (such as spring steels) are used to obtain a hardness of up to 700 HV after a martensitic hardening operation.

Ceramic and glass shot can be produced in a wide range of sizes (from 0.03 to 1.2 mm, depending on the application) and are used in compressed air shot peening as well as in sandblasting. With these materials, hardnesses superior to 1,000 HV can be achieved. Sintered zirconia oxide is the most commonly used ceramic material for shot peening springs.

Natural compounds with high hardness include corundum, which comes in colored variants and is used in jewelry, sapphire (blue/yellow), and ruby (red). On the Mohs scale, corundum (9) is just below diamond (10), which is the hardest naturally occurring stone.

Hardness

Since shot is used to peen inherently hard materials such as hardened steels (with high surface hardness) and pre-hardened steels (with a surface hardness of about 500-600 HV), it needs to be very hard and wear-resistant itself.

As explained before in the section on shot peening machines, shot is fired, hits the surface, falls to the bottom of the machine, is recovered, and its size is checked with a sieve before being reintroduced into circulation. If the shot falls below a certain size, it is discarded. A hard shot can withstand a certain number of impacts and be used multiple times before being discarded. However, with each impact, the shot undergoes a deformation that reduces its size and often causes it to break.

Typical hardness values are:

Steel: 600-700 HV

Glass/Ceramic: 700 to 1,000 HV, and for specific applications, up to 1,250 HV

Corundum: 600 to 800 HV

Parameters that Define Shot Peening:

This section is crucial for the know-how of each company. Therefore, we will not go into detail on the contents.

As mentioned, the effects of shot peening on springs can only be measured by estimating residual stresses, and thus with X-ray diffraction.

The measurement of the energy action of shot peening is empirically done with the Almen system, which estimates the potential of the shot peening machine.

The other parameters that vary from spring to spring are the batch size to be peened (charge quantity) and the cycle time.

Based on the performance of their own shot peening system, each spring manufacturer establishes their own shot peening cycles.

At Mollificio Valli, the combination of these parameters, supported by verification with microscopy and fatigue testing on peened and unpeened springs, allows us to guarantee the fatigue performance of the springs and the maintenance of such performance over time, in various batches.

The overall knowledge gained over the years and the use of high-resolution digital investigation tools contribute to the development of a concrete methodology in defining shot peening cycles and controls.

The control of the shot peening process, like any other process, can only be achieved by defining measurable parameters that can be monitored in real-time or in a preventive manner.

At Mollificio Valli, we have a quality control department that manages and monitors the shot peening process by performing regular checks on:

• Incoming shot checks;
• Verifying shot peened springs under a microscope;
• Conducting fatigue tests on shot peened and unpeened springs;
• Measuring shot peening machine efficiency through periodic Almen tests;
• Monitoring parameters of eddy current devices installed in-line on our coilers to detect and discard material sections with surface defects.

If shot peening is a crucial factor in the fatigue resistance of the spring, for particularly demanding applications in terms of life cycles, Mollificio Valli is ready to collaborate with you and provide over thirty years of expertise.

Angelo Passarotti

He graduated in electronic engineering from the Milan Polytechnic in 1992. Since 2000, he has been working at Mollificio Valli as technical sales manager.

Over the years, he has acquired extensive experience in the calculation and technical aspects of spring production.

He has always been passionate about mathematics and statistics, and has had the opportunity to apply his knowledge in statistical control techniques, metrological aspects and in general in the practical field of problem solving and continuous improvement.