Turning to the tube

Wasting high grade stainless steels in inefficient production processes is expensive but machining hollow bar can reduce costs. Ed Hill visited Sandvik Material Technology to see the benefits.

Wasting high grade stainless steels in inefficient production processes is expensive but machining hollow bar can reduce costs. Ed Hill visited Sandvik Material Technology in Halesowen to see the benefits.

Sandvik Material Technology's machining day was staged to promote the introduction into the UK market of its duplex stainless steel, Sanmac SAF 2205 in hollow bar lengths. The metal has highly resistant corrosion properties, good mechanical strength and increased machining and weldability performance making it ideal for applications in the oil and gas sector, automotive, food and marine as well chemical environments.

Sandvik says its Sanmac materials can overcome some of the machining difficulties that austenitic and duplex stainless steels can create such as work hardening and built up edges on the cutting tool.

Time trial
 
In tests carried out by Sandvik and an Italian valve manufacturer machining hollow bar SAF 2205, an increase in machining productivity of 224% was achieved. The trial involved manufacturing 20 trunnion ball valve seats, a typical batch for the company, used in the oil and gas sector.

Duplex material grade solid bar and hollow bar were used with the required sequence of machining operations undertaken to complete the finished component. The operation included, drilling, facing, semi-roughing, external and internal finishing and axial grooving. The process also involved manually rotating the workpiece so the rear face of the component could be machined.

In total, seven machining operations were necessary to complete each component from solid bar, including the initial drilling of the bore. By comparison only five machining operations were necessary to produce the component from SAF 2205 hollow bar.

The solid bar was 140mm diameter whilst the hollow bar had an OD of 140mm and an ID of 100mm. The trial used cutting data provided by Sandvik Coromant.

The time taken to machine the component from solid bar was just over 18 minutes while the hollow bar took just over five minutes. The final overall dimensions of the parts were 138.7mm by 38mm with a 101mm bore. Unsurprisingly, the reduction in the amount of metal that had to be removed also led to increased tool life of the cutting insert.
Rinaldo Brivio, productivity manager at Sandvik, comments: “Having customers that are open to discussions and recommendations on improving production processes means that we can advise and help with tests such as this, clearly demonstrating the process benefits in real time.”

At the event in Halesowen, Mr Brivio demonstrated how cycle times could be reduced using hollow bar of 90mm OD diameter by 200mm length. He explained the cost savings created by reduced cycle times are considerably more compared to lower material prices or increased cutting tool life.

“We wanted to revaluate the cost of the material and tooling because if we change the production process we change the fixed overall cost. If we can reduce the cycle time by just 5% it makes an enormous difference.”
His research revealed that the total cost per part from a batch of 100 components made from SAF 2205 solid bar, allowing for factors such as insert costs, machining centre costs, workshop costs and scrap amounted to €63.60 (approx £53.43). From hollow bar the total cost per part was €46.56 (approx £39.11).

Mr Brivio also assessed different speed and feed rates using a Sandvik Coromant insert to create varying chip breakage results and metal removal rates when using SAF 2205 on a Mazak turning centre.

Sandvik Sanmac SAF 2205 hollow bar is available in sizes from 32mm to 275mm outside diameter and comes in standard lengths of 2m to 6m. However, Sandvik offers a cutting service so material can be supplied at the optimum length to suite the customer's machine operation requirements.

Benefits included

Sandvik says that unlike other stainless steels, combining hollow bar and Sanmac stainless steel means cutting speed and feed rates can be increased. Tool wear also decreases as a result of the non-metallic inclusions within the material. Sulphides and oxide elements improve chip breaking and reduce tool wear without affecting the material's anti-corrosion and strength properties. As a result, engineering and machining companies can make significant cost savings by using extruded hollow bar with predefined outside and inside diameters.

Jan Haraldsson manager R&D, metallurgy at Sandvik, explains how the company had developed its duplex stainless steels down the years to achieve improved performance: “You can't make metal alloys without inclusions but you can control them as much as possible to get the desired result you want. Most of our customers need anti-corrosion properties but they mainly require the mechanical strength of the material. This means you can design components with thinner walls for example. With Sandvik you will have consistent quality. With our Sanmac stainless steels, including SAF 2205, we believe we have good consistency, good machinability, and weldability that make the product superior to similar materials in the marketplace. ”

Sandvik Material Technology has also created an app which design engineers and workshop operators machining holes in the centre of solid stainless steel bar can download to estimate the possible savings by switching to Sanmac hollow bar.

Andy Sharpe primary product manager at Sandvik affirms: “The material supplied as hollow bar in this grade is relatively new in the UK but there are there are certain applications to which it is well suited and we now have large stocks in our distribution centre. There are a number of projects coming out of the oil and gas industry at the moment and as well as supplying the main contractors in that industry, we are also serving the machine shops and subcontractors supplying into the sector.”

Sandvik Material Technology
www.smt.sandvik.com

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