Blocky Retained Austenite
Island-like and irregularly distributed austenite regions between martensite or bainite needles that tend toward undesirable deformation induction under mechanical loading.
Blocky retained austenite is austenite that didn't fully transform during cooling and settles as connected areas between martensite or bainite needles. The problem: when the structure is later loaded, this austenite can flip and cause local stress peaks, increasing crack susceptibility. Also, you get uneven hardness in the weld, making the joint unreliable. To avoid this, you must control cooling so the transformation runs cleanly. Don't let the weld cool too fast, but hold it at temperature in the critical range long enough for the austenite to distribute evenly. If you have problems, a subsequent heat treatment helps to complete the transformation.