CONTROLLED SHOT PEENING: METHODS AND PRECISION

CONTROLLED SHOT PEENING: METHODS AND PRECISION

Let’s start by talking about the parameters that define the shot peening, whose control should guarantee the repeatability of the process. There is therefore talk of controlled shot peening.

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

It is clear that the goal of these parameters is the possibility of obtaining a residual compressive stress, is as uniform as possible, on all the springs of the shot-peened batch and for all the shot-peened batches over time, which represents the ultimate aim of the shot peening.

X-ray diffraction analysis in the shot peening process

From a scientific point of view, the only internationally recognized method for measuring residual stress is X-ray diffraction. Specifically, we are talking about X-ray interferometry that is performed through a Specifically, we are talking about X-ray interferometry that is performed through a diffractometer, a machine that creates a beam of X -rays, which is conveyed on the surface to be measured. The wavelength of X-rays is similar to the interatomic distance of the atoms of the metal lattice. When the beam hits the surface of the metal it is reflected by the external surface of the metal and refracted by the crystalline planes that are located deeper.

APPLICATION OF BRAGG’S LAW IN SHOT PEENING

The first to study this phenomenon at the beginning of the last century were the Braggs (father and son), who developed Bragg’s law, which governs X-ray interferometry. In detail, Bragg’s law is:

nλ =2dsin(ϑ)

Where:

  • ϑ (theta) is the angle that the outgoing beam makes with the crystalline plane
  • λ (lambda) is the wavelength of the radiation
  • d is the distance between two adjacent planes
  • n is a positive integer and defines the multiple of the wavelengths

controlled shot peening - X-ray diffraction

The path between the ray reflected by the surface and the one refracted by the underlying crystalline plane at a distance d depends on d itself and on the angle ϑ, which in turn is linked to α by the relation α+ ϑ=90°.

Bragg’s law defines the link between the distance of the crystalline planes (d) and the characteristics of the beam (ϑ, λ). If I know the angle of incidence and the wavelength I determine the distance d, based on the interference pattern that the diffractometer creates. In the diffractometer I have an emitter part, which “shoots” the X-rays and a receiver part which collects the reflected/refracted rays and creates the diffraction pattern using software.

THE ROLE OF DIFFRACTION IN THE OPTIMIZATION OF SHOT PEENING

Diffraction is a typically undulatory phenomenon, where electromagnetic waves, such as X -rays are, spread in space with a sinusoidal pattern. Assuming the existence of a beam of sinusoidal waves, when they impact the surface to be measured, they are reflected (external surface) and refracted (crystalline planes under the surface). These waves are captured by the receiver and, depending on the path taken, the waves are in phase, in phase opposition, plus all the combinations between these two opposite states. When the waves are in phase they add perfectly (constructive interference), on the contrary when the waves are in phase opposition they cancel out (destructive interference. After processing by the software, a diffraction pattern is created, which presents a series of peaks, where the interference is constructive, that is where the reflected and refracted wave whose path has lengthened by a finite multiple of the wawe length of the ray sent to the surface. This elongation depends on the distance of the crystalline planes (d) and on the angle of incidence of the ray on the surface to be measured (ϑ)

controlled shot peening - Constructive interference

Constructive interference

controlled shot peening - Destructive interference

Destructive interference

If I know d for a non-shot peened material and I always measure d on the same shot peened material, I note how much the crystalline planes surface have moved closer, due to the compressive effect of the shot peening. This is then transformed into a compressive stress (Mpa).

controlled peening - Interference

Interference figure

In the image above an interference pattern is visible, in which the peaks correspond to the position of the crystalline planes in relation to double the angle that the emitter forms with the surface to be inspected (2 ϑ).

MEASUREMENT OF THE STATE OF RESIDUAL VOLTAGE

The chain of considerations that leads to measuring the state of residual stress induced by shot peening is quite complex. The sequence is as follows.

  1. 1. The peaks on the shot-peened and non-shot peened material are measured. The state of stress of a shot peened surface causes an increase in the angle (2 ϑ), for the same wavelength (λ). For convenience, instead of ϑ, it is preferable to measure ψ, which refers to the normal direction of the surface to be measured. controlled peening - residual voltage
  2. The angular distance between two peaks corresponding to the same crystalline plane defines the deformation ε, which is linked to the tension/stress σ via the well-known Hooke’s law (σ=E* ε).
  3. The problem must be analyzed in three dimensions, therefore the stress state includes 3 ε (εx, εy, εz) and 3 σ (σx, σy, σz).
  4. By combining the 3 equations along the three directions x,y,z, a numerical relationship is obtained which defines the stress in relation to the gradient of variation of the elongation ε, which in turn is related to the angle of incidence on the surface to be to measure. The relation also contains E (Young’s modulus) and ν (Poisson’s ratio). To give an image of this somewhat cryptic definition, see an example here below.

controlled peening - graph

THE LIMITS OF DIFFRACTOMETRY IN SHOT PEENING CONTEXTS

The limits of X-ray diffractometry are linked to the characteristics of the surface to be analysed. If the surface is flat there is no problem, but if the surface is curved, as happens in the round wires used for compression springs, the curvature negatively affects the reliability of the diffraction pattern. This phenomenon makes it virtually impossible to perform reliable X-ray analysis for wires smaller than 2.5 – 3 mm in diameter.

The costs linked to carrying out a diffractometric analysis and/or the investments necessary to create and manage a plant make this verification tool little used in the industrial sector. While it certainly remains useful in fracture analyzes and metallographic tests, especially in flat products.

Therefore, for the shot peening of compression springs, not being able to take advantage of a practical and quick investigation tool for evaluating the shot peening effectiveness, empirical parameters are defined which are used to monitor the shot peening process.

These parameters are:

  • The Almen degrees.
  • The degree of coverage.
  • Diameter, shape and hardness of the shots.

Understanding Almen grades in the context of shot peening

Let’s now talk about the Almen degrees, the Almen test is an empirical test to try to measure the impact energy of the shots shot from the shot peening machine. Given that any theoretical approach clashes with problems of measuring the variables involved, it is preferable to use a practical test, trying to standardize the elements involved, to guarantee their repeatability.

THE ALMEN TEST: A STANDARD IN THE EVALUATION OF SHOT PEENING

To perform an Almen test you need the following elements:

  • 1. Certified specimens. These are metal rectangles with specific thickness, size and hardness. The material is a hardened SAE1070 CRS, equivalent to a C67 EN10132-4, with hardness between 44-50 HRC
  • 2. A specimen holder, which is used to constrain the specimen in 4 points at pre-established distances.
  • 3. A comparator to measure the deflection of the specimen, in micro inches (25.4 x 10-6 mm).

In practice, the Almen test consists of inserting a specimen mounted on the specimen holder, into the shot peening machine, keeping it in a fixed position, exposed to the flow of the shot. The shot peening time is then progressively increased. After each shot peening cycle, the deflection of the specimen is measured.

The effect of the impact of the shots on the specimen is actually to make it curve (deflection). Intuitively it is understandable that there is a proportional relationship between the inflection height and the peening time. The logic underlying the phenomenon suggests the according to which the more we shot peen, the more we deform and therefore greater is the inflection.

This is true in a certain range, because after a certain value the inflection stops growing and stabilizes.

What is explained in words is clearly visualized with the so-called saturation curve.

controlled shot peening - saturation curve

saturation curve

As we can see, the graph in the first part is almost linear and then begins to curve, until it actually reaches a constant trend. By convention, the measurement of the deflection which represents the intensity of the shot peening is the deflection value (deducible from the curve) which represents 98% of the total deflection.

As mentioned, the specimen plates all have the same size, but different thicknesses.

DIFFERENT TYPES OF SHOT PEENING MACHINES AND THEIR ADAPTABILITY

It is intuitive to understand that there are “small” and “large” shot peening machines with consequent very different impact energy. It follows that the thicknesses of the specimens increase in relation to the energy size of the shot peening machine.

In practice the specimens have 3 thicknesses, which characterize the Almen degrees types (N,A,C).

controlled shot peening - thicknesses

Almen N degrees are for small shot peening machines, compressed air ones with ceramic or glass shots.

Shot peening machines with steel shots for springs with wire diameters no more than 9-10 mm, use grades A.

Heavy” shot peening machines use C degrees.

For shot peening machines with constant speed turbine, the Almen degrees identify the shot peening intensity of the system. Inverter or compressed air shot peening machines with variable pressure are able to cover a series of Almen degrees in relation to the speed of the turbine or the pressure of the system. The shot peening intensity depends on the kinetic energy of the shot peening flow. If we change the speed of the shots the kinetic energy changes and therefore the intensity. We can change the intensity only in systems with variable shots speed.

The Almen degrees define the energy size of the shot peening system, but the definition of the shot peening times is different. Shot peening times are part of the know-how of each company and are defined by seeking the right compromise between the size of the charge (kg) and the achievement of an adequate degree of coating, avoiding the phenomenon of overpeening.

Evaluation of the degree of coverage

The degree of coverage, as the word itself suggests, measures the extension of the shot peened surface. It is verified visually with the help of a microscope with a magnification of at least 50x.

Once the inspection surface has been defined, the extension of the marked surface (by the shoots) in relation to the inspected surface is verified. The goal is to have a coverage degree of 100%, that means that the entire surface is impacted by a shot.

We are talking about surfaces covered at 150-200%, when overlapping prints of multiple shots are visible.

The degree of coverage is directly dependent on the shot peening time, the longer the shot peening time, the greater the coverage, because the probability that one or more shots impact on the affected surface increases.

PREVENTION OF OVERPEENING FOR CONTROLLED SHOT PEENING

The choice of time is crucial to guarantee adequate coverage. Be careful not to overdo it, because the downside is the phenomenon of overpeening, that is an excessive peening. In this case the shot peening has more deleterious effects than beneficial ones, the hyper-impacted surface tends to flake and notches are formed which are potential trigger points for cracks.

Here below there is an explanatory graph of the effect of overpeening on aluminum for aeronautical uses. The number of fatigue cycles in the ordinate refer to plane bending cycles, measured on an interlocking plate, stressed in alternate bending, applying a load to the free end.

controlled shot peening - fatigue resistance

The percentages refer to the degree of coverage.

THE ESSENTIAL KNOW-HOW IN CONTROLLED SHOT PEENING

Shot peening time is a technical-economic compromise. It must be sufficient to reach the coverage requirements (>>100%), avoid overpeening and be economically advantageous, that is adequately short to optimize energy and grit consumption, without compromising the achievement of the set objective.

The choice of timing is part of the know-how of every company. At Mollificio Valli, shot peening times are the result of an analysis that covers all the objectively measurable parameters of controlled shot peening:

  • Degree of coverage (optical microscope assisted by specific software).
  • Check of incoming shots for size and hardness.
  • Fatigue tests on shot-peened samples, using strokes designed to induce premature failure, based on the verification carried out with the Goodman diagram.
  • Periodic control of the beam orientation.
  • Periodic verification of the Almen degrees of the plant.

Shot selection: a crucial aspect of shot peening

The grit used to carry out the shot peening is the physical agent through which the conversion of kinetic energy into deformation energy occurs, which corresponds to the sub-surface compressive state that we want to obtain on the springs.

Maintaining the geometric-dimensional characteristics of the shot is among the key factors to guarantee the repeatability of shot peening. This aspect is guaranteed primarily by the vibrating screen system incorporated into the shot peening machine, which excludes damaged or worn shot from recirculation, the size of which falls below a set value.

Above all, there is the quality of the shots used, in terms of minimum hardness and effective dimensions. These factors are monitored with adequate entry controls.

The difference between shot peening and controlled shot peening, which is the process in use at Mollificio Valli, lies in the periodic monitoring of all the factors that contribute to achieving shot peening. Even those that apparently seem to remain constant.