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#768 - 05/16/11 11:51 AM Shot hardness compared to target hardness
Jack Champaigne Offline


Registered: 04/20/99
Posts: 320
Loc: Mishawaka, IN, USA
[From unidentified user]
hi,
I try to identify the influence of the difference of hardness between the piece and the shot. In my case there is a big difference: the hardness of the shot is 610HV and the hardness of the piece is 400HV. I ask when the difference is acceptable; I know that the shot should be harder than the piece (no more information about how much).
thanks

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#769 - 05/17/11 11:36 AM Re: Shot hardness compared to target hardness [Re: Jack Champaigne]
Socrates Offline


Registered: 02/20/06
Posts: 108
This is a very important question. We need the component to behave plastically and the shot to behave elastically. Then the component is simply dimpled by the shot particles and the particles themselves are not plastically deformed. Plastic deformation occurs when the yield strength is exceeded. The higher the hardness the higher is the yield strength. It follows that the shot must be harder than the component in order for the shot to behave elastically and the component to behave plastically.
Having decided that the shot must be harder than the component the next problem is to select appropriate shot. The harder the component the harder must be the shot. That is why manufacturers produce ‘high hardness’ grades of, for example, steel shot. There is no specified difference in hardness ( component versus shot) that has to be applied. It would be unreasonable to expect shot peeners to keep switching from one hardness grade of shot to another in order to handle components with different hardnesses. They just have to make sure that none of the components to be peened have a hardness greater than the shot currently being used. If several machines are available they might only have different shot sizes rather than different hardness grades. Extra high hardness shot would only need to be employed if the size of the order justified the costs involved (cleaning out previous shot, re-charging, new intensity determinations, etc.).

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#770 - 05/25/11 04:39 PM Re: Shot hardness compared to target hardness [Re: Jack Champaigne]
Pete Bailey Offline
Member

Registered: 01/12/05
Posts: 9
Loc: Hamilton, OH
(1)If the question is a general one, I agree with the words above. If the question relates to a fatigue critical part, then tests should be run with different shot sizes, intensities and coverages as well as shot hardness under service conditions of temperature, stress, etc.
(2)Recognize that normal hardness cast steel shot work hardens in use beyond the delivered HRc 45-52.
(3)Shot softer than the part can still produce surface compression - hardened gears, for instance

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