Alloy Variation Solves Metal Flow Problem in Staking Components for Emission Controls

Case Studies

When a metalworker is having problems with an alloy that seemed just right for the job, a variation of that material might provide all the attributes originally expected, and more.

That was the case with a fabrication dilemma shared by AAA Industries, Inc., a Detroit producer of OEM screw machine parts, and Siemens Canada, Ltd., Chatham, ON, manufacturer of automotive emission control devices.

AAA Industries was mass producing valve seats from Type 416 stainless steel for Siemens to incorporate in the emission controls it was making for a major automotive manufacturer. AAA had no problems machining the valve seats, but struggled to meet surface finish requirements.

Seimens, however, had major problems staking the AAA valve seats into its cast iron EGR (exhaust gas recirculating) valve base. The difficulties came at an inopportune time because the company was trying to gear up a new assembly line for full production.

The stainless steel would not flow as required during the critical staking operation. When press pressure was increased to facilitate the severe cold forming, either one of two pins supporting the EGR base would break after about every 100 parts produced. Each breakdown would cost the plant 20 minutes in press downtime for pin replacement. Even worse, the EGR bases virtually exploded a few times from the pressure applied, causing serious interruptions.

Material Change

The screw machine shop requested technical assistance from Carpenter Technology Corp., Reading, PA, who was supplying the stainless Type 416 bar stock. A regional metallurgist from Carpenter's Detroit service center visited and conferred at length with both metalworking plants, then recommended that they consider changing to Carpenter stainless No. 5-F.

Siemens had specified use of Type 416 stainless because of its expected machinability, wear resistance, good behavior under impact loading and corrosion resistance. The steel, however, did not offer good staking properties.

Carpenter stainless No. 5-F is a modification of Carpenter's martensitic stainless Type 416. No. 5-F is a ferritic stainless steel, with a specially balanced composition, that is designed for optimum machinability with corrosion resistance. It is essentially non-hardenable, normally furnished in the annealed and cold finished condition at a hardness of approximately 200 Brinell.

After running production trials with Carpenter stainless No. 5-F and experiencing gratifying results, Siemens decided to change its print specifications to the alternative alloy.

In the current production sequence, AAA Industries produces its stainless steel valve seats on a Davenport multi-spindle screw machine at the rate of approximately 750,000 per year. The valve seats are typically a nominal 1/2" long by 1/2" dia. with a tapered inside diameter which, at the narrow bottom, provides seating for the valve head.

The shop, on the Davenport, forms a large and small diameter on each valve seat, drills the angled ID, cuts off to length, then chamfers on another machine. It has improved surface finish in the lower seal area from 60 micro finish, with the Type 416 stainless, to 30 micro finish, obtained with Carpenter stainless No. 5-F. The better finish has assured a tight seal between the base of the valve seat and the face of the valve head, thus improved part performance.

Siemens has staked the valve seats from AAA Industries on a fully automated, specially designed eight-ton hydraulic press. The valve seats and cast iron EGR bases have been loaded on a pallet, shuttled into the press and staked automatically.

A staking tool made of T-15 high speed steel with titanium coating has been used to press the valve seats into the EGR base.

When pressed into the valve seat, the four-point tool displaces material in a radial direction, filling a circular groove 2.5 mm deep by 1.5 mm wide that had been machined into the casting before the staking operation.

The metal filling the circular groove provides an interference fit with the base and secures the valve seat at four points. With the valve seat firmly secured, the valve head can make precise, efficient contact with every stroke.

Staking Problem 

Until the switch to Carpenter stainless No. 5-F, metal flow, hence stakability was always a key issue. In the beginning, Siemens asked AAA Industries to anneal the original Type 416 stainless so that it might be easier to cold work. That procedure added cost, resulting also in parts of inconsistent hardness and valve seats that wore excessively.

The EGR base support pins, that broke under increased press loads, could not be enlarged to increase strength because they had to go through the 3/8" holes provided for engine assembly. Those holes, obviously, could not be changed.

Results Gained 

With the switch to a more suitable alloy - Carpenter stainless No. 5-F - Siemens was able to correct its fabrication problem before its assembly line went into full production. The company has been staking up to 10,000 parts daily, at the rate of one every five seconds. This is several times more than the best productivity rate achieved with the Type 416 stainless. The production line, in fact, has been running for three months without a hitch in its staking operation.

With the steel flowing as required in the staking operation, Siemens has been able to tightly secure the valve seat in the base of the EGR, and assure precise contact between the valve seat and valve head. That helps provide good performance for its emission controls. In addition, valve seat wear has been negligible.

AAA Industries also has experienced successes with the alloy change. It has gained a 100% improvement in surface finish (from 60 microfinish down to 30 microfinish), which is vital to the interaction between valve seat and valve head. In addition, it has been getting longer tool life, now maintaining tools every two days instead of daily.

Nominal analysis of Carpenter Stainless No. 5-F is: carbon 0.10% max., manganese 1.00% max., phosphorus 0.06% max., sulfur 0.30% min., silicon 1.00% max., chromium 13.00/14.00%, nickel 0.50% max., balance iron.