Chemical composition (nominal) %

Forms of supply

The following range of strip thicknesses and widths can be supplied as standard. Other dimensions can be offered on request.

* Depending on requested tensile strength.

The cold-rolled strip can be supplied in coils, bundles, on spools or in cut lengths.

Sandvik Bioline 1RK91 can also be delivered as wire/bar.

Mechanical properties

The possible ranges for the mechanical properties both in the cold-rolled and aged condition are indicated below.

The strength level after ageing depends on the amount of cold deformation and therefore also on the final dimension.

1 MPa = 1 N/mm2
a) Rp0.2 correspond to 0.2% offset yield strength.

Examples of strength values for the heat treated (aged) condition are shown below. As the true values depend on product form and production route, the exact value for a specific product or application must be determined in each case. Please contact Sandvik for further information.

At elevated temperatures

The values represent testing on material cold worked to a tensile strength of 1650 MPa and subsequently aged at 475°C to 530°C for 4 hours.

Physical properties

The physical properties of a steel are related to a number of factors, including alloying elements, heat treatment and manufacturing process. The data presented below can generally be used for rough calculations.

Density: 7.9 g/cm3, 0.29 lb/in3
Resistivity: cold worked: 0.97 μΩm, aged: 0.83 μΩm

1) For material in heat treated (aged) condition

1) For material in heat treated (aged) condition

Thermal expansion

Average values in temperature ranges. The steel grade has a coefficient of thermal expansion close to that of carbon steel. This gives it definite design advantages over normal austenitic stainless steels.

1) (x10-6/°C)

1) (x10-6/°F)

Modulus of elasticity

The E-modulus depends on dimension and amount of cold deformation in the material. For bar form there is no data available, but for wire and strip E-modulus between 185 - 200 X 103 MPa have been achieved.

Corrosion resistance

Sandvik Bioline 1RK91 has a corrosion resistance comparable to ASTM 304L or ASTM 316L depending on condition and environment.

Pitting and crevice corrosion

The Critical Pitting Temperature (CPT) has been determined using electrochemical CPT testing at 300 mV in NaCl solutions of different concentrations at pH = 6.0, ground test samples (600 μm). All results are average values from six measurements.

Figure 1. Critical pitting temperatures (CPT) for Sandvik Bioline 1RK91, ASTM 304 and ASTM 316 at varying concentrations of sodium chloride. Potentiostatic determinations at +300 mV SCE, pH = 6.0

General corrosion

Figure 2. Isocorrosion diagram for Sandvik Bioline 1RK91, ASTM 304L and ASTM 316L in stagnant sulphuric acid. The curves for ASTM 304L and ASTM 316L and the dot for Sandvik Bioline 1RK91 represent a corrosion rate of 0.1 mm/year

Heat treatment

In the annealed condition, Sandvik Bioline 1RK91 has an austenitic microstructure. To be able to precipitation harden the material and take advantage of the remarkably high ageing (tempering) effect, the matrix has first to be hardened and, thereby, partly transformed to martensite. There are two ways of obtaining the necessary martensitic matrix in Sandvik Bioline 1RK91.

Cold-rolled condition

The most common way, as in other metastable austenitic stainless steels, is to cold roll the material, whereby deformation martensite is formed. The difference is that the ageing effect is much higher in Sandvik Bioline 1RK91 than in most other stainless steels. For optimum strength, the ageing should be made at 525°C (977°F) for 1 hour. Some examples of the ageing effect are given in the table.

Annealed condition

With Sandvik Bioline 1RK91 there is another way to obtain a martensitic matrix - by an isothermal treatment at a subzero temperature. This gives an opportunity to use the grade in the soft annealed condition for severe cases of forming and then utilize its ability to still reach a comparably high level of hardness after heat treatment. The required heat treatment cycle and the resulting properties are:

Austenitizing: 1200°C (2190°F) for 5 minutes in a protective atmosphere like argon, hydrogen or vacuum.
Isothermal treatment: -40°C (-40°F) for 24 hours
Ageing: 525°C (997°F) for 1 hour

Weldability

The weldability of Sandvik Bioline 1RK91 is good. Suitable welding methods are TIG, MIG and MMA. It can be welded without filler metal (autogenously) using the TIG process, but filler metal is preferable. For TIG and MIG welding, Sandvik or 19.12.3.L can be used, or if a higher strength is desired, Sandvik 22.8.3.L. For MMA, the corresponding electrodes Sandvik or Sandvik are suitable.

The martensitic content in HAZ of the material decreases after welding resulting in a typical annealed microstructure with an austenitic matrix and a small amount of ferrite. This means that the tensile strength will be lower for the weld compared with the high strength base material. Therefore, welds in Sandvik 1RK91 are not suitable for active parts of a construction, when extremely high strength is required.

Applications

In strip form, Sandvik Bioline 1RK91 can be used for hypo-tube application.

In wire form, Sandvik Bioline 1RK91 can be used - depending on the diameter- for surgical suture needles, blood lancets and dental tools (taps, reamers, screw drivers).