I often use Yafray together with Blender 3D for product design rendering. Both programs are free and good open source applications. The 3D rendering of metal objects as shown below was done with Yafray.
The followings are the Blender 3D material settings used on these metal pieces. I am no rocket scientist and thus am not fully aware of the effects of all the individual settings and parameters. These settings are experimented and tried from purely trial and error renders. Perhaps there are more than one way to get the same effects.
For Diffuse Shade, Minnaert was used. For Specular Shade, Blinn was used. Spec and Hard value should be set low or to zero. This is because metallic surfaces typically do not exhibit specular highlights.
Under the Mirror Trans, the Ray Mirror was turned on. The higher the Raymir value, the more reflective the metal will appear. For a more matt look, the Raymir value should be keep low.
For the rendering, a HDR light probe file was also loaded in to give better contrasting reflections. From my experiments, using HDR instead of a normal image map (for environmental mapping) will significantly enhance the realism of the renderings.