A method of controlling strip temperatures of two joined metal strips of dissimilar gauges but requiring essentially the same heat treatment temperatures, is described. The normal processing temperature is higher than the temperature to which it is desired to heat the webs. The higher processing temperature is reduced to a lower temperature in response to a change in the gauge of the webs passing through the chamber in which the webs are heat treated. This lower temperature is such that the heavier gauge web will not be underheated and the lighter gauge web will not be overheated. The line speed at which the juncture of the webs passes through the chamber will simultaneously be lowered in correlated relation to the reduction of the temperature within the heat treatment chamber until such time as the juncture passes from the chamber so that the processing conditions of time and temperature can be readjusted to accommodate heat treatment of the new web.