Below 1000°C a sufficient wetting by means of Cu/Ag-Solder doesn’t take place because of the chromic oxide layer on the surface of austenite high-grade steel. Only at higher temperatures the chromic oxide layer tears up and inside those rips the wetting takes place.
To create a sufficient wetting at around 800°C the solderingarea usually is manually furnished with an about 5-20 µm thick galvanic nickel or copper layer. Due to galvanical prepraparations the oxide skin is skimmed by pickling and afterwards never develops again under the galvanical applied nickel or copper layer. Wrong soldering parameters, however, can cause the Cu/Ag-solder to dissolve the thin Cu-Layer.
Since this doesn’t happen with nickel high-grade steel pieces should be nickelplated.
Similar cases exist with other materials that are difficult to solder (like Glidcop as sinter material, copper-chrome-zircone (ELBRODUR), copper-beryllium, tungsten) because those, too, are easier to galvanise in an adherent way instead of soldering them.