We would all like to offer as much assistance in each others welding questions and problems, that is what makes forums so successful and enjoyable. Put all the facts out there in the beginning and a quick and accurate responce will develop. What is the grade of 4XX series, ferritic will not respond to heat treat for hardness, martensitic will and will definiately require a degree of preheat (how much and how long will be determined by , so far, unknown variables. Next consideration is what is expected out of the weld ???? If the weld has to respond to heat treat that the use of a 3XX series filler is out, waiting to see more info ........ and by the way, there are 5 and 9% chrome alloys out there the do require preheat and can be heat treated for hardness, very common in the tool and die industry.