Thursday, February 14, 2013

SAE 841- Bar Stock vs Bearings

Day in and day out we have customers who contact us for SAE 841, Sintered Bronze material.  The call will start off by the customer giving an order for pieces of 841 Bar Stock or Solid Bar material.

Then somewhere in the conversation the customer will say that they need a Signed C of C per ASTM B438 and then the dreaded statement comes....."I will be making Bushings".  This is where we have to stop the customer and go into the usual spiel about the ASTM B438 specification.  Now before I continue, here is our disclaimer**:  We are not expertsOur sales team has just been burned enough times that it has forced us to learn and understand this crazy spec. 

 The sales person then proceeds to explain to the customer that the ASTM B438 specification is for Bronze-Base Materials used in Powdered Metal Bearings.  The key word being -- Bearings.  This covers Bearings / Bushings or Thrust Washers. 

841 Facts

Here is the deal.  Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of five steps: 

(1) Blending - The part-specific powder is created by mixing the correct amounts of metals and lubricants to produce the physical and mechanical properties of the finished product.

(2) Molding - the powder is compacted into the desired shape using compressive forces. The part shape is created by compressing the correct blend of materials inside of the part-specific tools.

(3) Sintering - Through the application of heat, sintering permanently bonds the individual metal particles that have been compacted together through the molding stage.  This process is instrumental in providing the majority of the mechanical properties of the final product.

(4) Sizing - Sizing, sometimes referred to as "coining," is the final pressing of the fully sintered part.

(5) Oil Impregnation - Under vacuum, customer-specific oil products are impregnated into the remaining porosity of the completed part.

The Problem

Notice in Step #3 that the Sintering process is the instrumental phase in providing the majority of the mechanical properties of the final product.  The ASTM B438 (05, 08 & 13) specification covering Sintered products applies to Bearing/Bushings/Thrust Washers.  These shapes have specific radial crushing strengths (minimum and maximum's), Impregnation Efficiencies and Impregnation Density's that are unique to the shapes dimensions and sizing.  

So why does that matter to the guy buying Solid Bar Stock and machining it into a Bushing? Consider this, let's say that I buy a piece of 841 Bar Stock, 1" ID x 3" OD x 6-1/2" long.  This piece of bar stock has a specific radial crushing strength, impregnation efficiency & density.  Now when you take that piece of bar stock back to the shop and machine it down to say 1-1/4" OD x 2-1/8" OD, you have now changed the radial crushing strength, impregnation efficiency & density.  Once the customer makes that "structural" change we, Atlas Bronze, can no longer say that that material is certified to the ASTM B438 specification.  It is believed that when you "ream" or "bore out" the ID of a piece of 841 Bar Stock you close the pores and alter the impregnation of that piece of bar.  At that point, Atlas can no longer say that the Signed C of C that lists specific physical & mechanical properties apply.  


Buy what you need.  If you need a Bushing, order a Bushing.  If you need a Flanged Bushing, get a Flanged Bushing.  Yes, you might pay a little more if the parts are not standard sizes...but in the long run, you know that the Physical & Mechanical Properties as outlined in the ASTM B438 spec will be in conformance to what you purchased. And most importantly read the specifications, ASTM B438-05,  ASTM B438-08 & ASTM B438-13.

 If you need a copy of one or all of these specs you can download them by visiting: and type in ASTM B438 in the search bar.  

**DISCLAIMER:  While Maranatha Now, Inc. dba Atlas Bronze strives to make the content on this blog as informational and educational as possible,  Maranatha Now, Inc. dba Atlas Bronze makes no claims, promises, or guarantees about the accuracy, completeness, or adequacy of the contents of this blog, and expressly disclaims liability for errors and omissions in the contents of this site.

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