For the mainstream 304 and 316 austenitic stainless steel hot runner bolts, the conventional solution treatment process involves heating the workpiece to 1050-1100℃ and holding it for 30-60 minutes, followed by rapid immersion in room temperature water for quenching and cooling. This type of austenitic stainless steel does not undergo martensitic transformation after solution treatment; during cooling, only a uniform and stable supersaturated single-phase austenitic structure is formed, without the large amount of residual stress accumulation caused by quenching phase transformation. After the workpiece is removed from the quenching bath and allowed to cool completely to room temperature for 15-30 minutes, subsequent thread finishing, surface polishing, chamfering, and deburring can be directly performed without any additional aging period. Machining at this time will not damage the already formed stable austenitic structure, nor will it cause post-machining springback deformation. The thread accuracy after machining can be stably controlled at grade 6H, fully meeting the installation and fitting requirements of the hot runner. In mass production scenarios, to further reduce the minute residual stress from quenching, some factories will extend the natural resting period by 12 hours after solution cooling before initiating subsequent processing. This waiting period is not mandatory; it's simply to allow the minute internal stresses of the workpiece to be fully released, preventing extremely slight dimensional deviations in subsequent precision thread machining. For hot runner bolts with ordinary precision requirements, this step can be omitted entirely, and machining can proceed immediately after cooling.
However, for hot runner bolts made of high-temperature alloys such as Inconel 718 used in high-load applications, machining cannot be carried out directly after solution treatment. The core strengthening mechanism of these alloys is aging precipitation. After solution treatment, an aging treatment must be performed at 720°C for 8 hours, followed by cooling to 620°C at a rate of 50°C per hour and holding at that temperature for another 8 hours. Only after the entire aging process is completed and cooled to room temperature can subsequent machining begin. If aging is skipped and machining is initiated directly, the strengthening phases precipitated during the subsequent aging process will cause minute dimensional deformations, directly compromising thread precision and rendering the bolt unsuitable for the precision installation requirements of hot runner systems.
It is particularly important to note that machining should never be forced while the workpiece is still at a high temperature after solution treatment. At this time, the material is in a highly ductile state, and machining will generate a large amount of new processing stress. Subsequently, under long-term operating conditions above 200°C in the hot runner, these stresses will slowly release, causing minor deformation of the bolts, eventually leading to loosening of the installation torque, or even stripping and breakage. Strictly adhering to the processing time points corresponding to the material is crucial to ensure that the final performance of the hot runner bolts fully meets the standards and is suitable for long-term high-temperature operation.
