in Vol. 19 - January Issue - Year 2018
Key Shot Peening Specifications Rewritten
The shot peening process provides benefits in materials performance in many industry sectors. Many of the specifications discussed are focussed on the aerospace sector and have the designation AMS – Aerospace Material Specification. However, two of the most important and ubiquitous specifications do not have the AMS designation as they carry-across all types of industry and are controlled as one of the SAE “J” specifications. These are the “J442 - Test Strip, Holder and Gage for shot Peening” and “J443 - Procedures for using Standard Shot Peening Almen Test Strip”. Both of these specifications have been rewritten and published in August 2017 to provide the most up-to-date methods in the control of the shot peening process.
Some of the notable changes in these specifications are listed below.
3.1.9 Pre-bow requirements are now defined for sub-size strips, conforming to the requirements for standard strips.
3.2 Test strip holder. The contact zone must be maintained to a flatness of 0.025mm or 0.001 inch. Also, it must be free of any deformed metal, such as burrs, that project above the surface. These requirements are necessary because any irregularity in the surface of the contact area can pre-stress the strip as it is screwed-down on to it. Note that the flatness requirement is more stringent than in previous revisions so users should check that their strip holders comply to the latest requirements.
3.2.3 Attachment of Sub-Size Strips specified as adhesive material or end screws. The main requirement being to use a consistent attachment method for processing and when undertaking the correlation to standard strips.
3.3 Provides requirements when using a scrap part as a test strip holder. Using a scrap part is good practice, as all the effects of part geometry will be taken into account when determining and verifying the intensity. Often, Almen strip holders are attached to the scrap part, but sometimes the strip is attached directly to the part surface. This paragraph specifies part hardness, screw positioning and allowable Almen strip lateral movement.
3.3.2 Provides requirements for an Almen gauge for sub-size Almen strips. Sub-size strips can be used to verify intensity in locations that are too small for full-size strips. Using “shaded strips” is an alternative method, which still uses the full-size strips.
Figure 1B This diagram has been added to define the dimensions for sub-size strips. Reference to sub-size strips have been included in the previous revision of the standard but more detail, comparable to that for full-size strips, is now being included.
Figures 4 & 5 These figures have been added to specify the design of gauging methods for sub-size strips.
3.3.5 Adds the requirement that pre-bow compensation must be used with sub-size strips due to low arc height values.
126.96.36.199 Describes the construction of saturation curves for sub-size Almen strips so that they can be correlated to standard Almen strip intensity requirements.
3.4.1 Describes how batch peening must use an Almen holder fixed in the shot stream and not a holder allowed to tumble in the batch. This is because tumbling in the batch can result in impacts on the surface of the strip from parts or the tumbling fixture.
3.5.1 Makes a clear requirement that the frequency of Almen strip verification tests shall not exceed 8 hours.
3.5.2 Clarifies that the strip verification value must be compared to the sat curve value for the same exposed time (the target arc height). This may be different to the determined intensity value if verification is undertaken at a time other than T.
188.8.131.52 With multiple strip positions, the verification test time may be anywhere between the shortest and the longest saturation time for the various saturation curves. The test arc height at each position shall be compared to the target arc height for that position at that exposure time. The verification must be within +- .038mm of the target value.
Section 4 gives a range of requirements and guidance for developing correlation curves when using sub-size strips and shaded Almen strips. These correlation curves must be developed before part processing so that correct verification may be carried-out. It also requires that correlation curves be recreated after any machine maintenance or critical part replacement on shot peening equipment.
These are some of the key points, but users must obtain full copies of J442 and J443 and undertake a full review to ensure conformance to all the requirements.
For questions contact firstname.lastname@example.org
by Paul Huyton,
MFN Course Director World Wide
more information at www.mfn.li/trainers