The hardness of needle tubing has been the subject of controversy for many years. The terms "full hard", "spring temper", and "number 3 temper" have been used interchangeably to specify needle tubing. These three terms have different meanings within the various medical and industrial markets. They even have different meanings between manufacturers. Needle tube manufacturers supply significantly "harder" tubing than industrial tube manufacturers for the same designation of hardness.

     Hardness is the measure of a material’s resistance to being scratched or penetrated. Hardness is achieved by cold working the tubing after it has been welded. The more cold work that is applied to the tubing, the harder it becomes.

     To meet stiffness specifications in the GG-N-196 and ISO 9626 standards, a certain amount of hardness is required. However, contrary to popular belief, increasing the hardness of tubing in the "needle temper" range has a minimal effect on the stiffness of the tubing. The greatest impact on the stiffness of tubing comes from changes made in the cross sectional area of the tubing. Figure 1 shows the effects of increasing the cold work (i.e., the "hardness") and the wall thickness (while keeping the outside diameter constant) on the resulting stiffness of the tubing. It is evident that stiffness is affected much more significantly by even small changes in wall thickness than by increasing the hardness of the tubing.

     Needle tubing has neither a hardness nor a tensile strength requirement in the current specifications. Frequently, purchasers of needle tubing will specify hardness and/or tensile strength, with little knowledge of the role these qualities play in the eventual performance of the tubing. The relationship between ultimate tensile strength and hardness is shown in the graph in Figure 2. Note that as the higher hardness values, hardness increases at a slower rate than the tensile strength. This explains why increasing the amount of cold work is effective in increasing stiffness to a point, but beyond this point, additional cold work has minimal effect on increasing stiffness.

     Increasing hardness reduces the ductility of the tubing. Special processing or fabrications such as swagging and flaring are work hardening operations that increase tubing hardness. Figure 3 shows the amount of flaring possible with tubing at different levels of hardness. Other fabrications such as swagging may follow a similar pattern. It should be noted that the many differences in tooling and techniques used by fabricators for both swagging and flaring will have varying results.

     Grinding is the most common fabrication applied to needle tubing. What range of hardness provides the most satisfactory grinding characteristics? K-Tube has defined a hardness range described as H3 to H5 as "needle temper", which has proven successful in point grinding for several years and millions of feet of tubing. (The full range of hardness from soft to maximum hardness is described as H0 to H5.) H2 has been used on occasion, with no grinding problems, but H2 is not considered to be within the normal hardness range for needle tubing.

     K-Tube recommends that fabrication requirements be discussed with the tubing manufacturer, so that the most appropriate specifications can be developed and incorporated for your application(s).