Study On Strengthening And Toughening Of CALDIE Steel By Heattreatment

This thesis studies CALDIE steel, the typical cold mold, and found the best heat treatment that suits CALDIE steel to solve the chip problems that easily arise from the work. The research adopts the methods of normal quenching treatment, Q-P treatment, Q-P-T treatment these three heat treatment to realize an improvement of the tenacity without an obvious decline of the hardness. The CALDIE steel structure and quality is also represented by XRD, OM, SEM, TEM and other analyzing methods with the help of X ray.When using isothermal quenching treatment at first, bainite organization is formed between 260℃ and 300℃, and the bainite ferrite is in the shape of a needle, and if it is formed under 320℃ then the feather like organization is up bainite. The test result of the sample XRD manifests that the remained austenite after the isothermal quenching treatment is between 17% and 24%, and the longer it is kept under this temperature, the less the remained austenite is, and the expressed hardness also declines, while the tenacity is harnessed. After the quenching treatment, the sample tenacity is 6.6J, which reaches the highest 11.8J after 5-hours keeping under 300℃, while the hardness dropped below HRC56 at this time.Next step is to stabilize remained austenite after q uenching by Q-P treatment, and the austenite accounts for 7% to 10% averagely through XRD analysis. The longer it is kept in the same temperature, the more carbon atom will spread from martensite into austenite to increase the amount of carbon in austenite. When the steel is under 400℃ and has been allocated carbon for 5 minutes, the carbon amount is the most, which is 0.94%. And because the carbon allocation is finished, the later lost carbide increases the consumption of carbon, during which the carbon amount drops to 0.87%. After the Q-P treatment, the hardness drops little, with an increase of tenacity, and after it is kept under 400℃ for 5 minutes, the tenacity reaches its highest level of 9.2J, and the hardness is above HRC58.In the end, after Q-P treatment, put the remained austenite back to fire of 550℃ for 1 hour, it dissolves and with an about less than 5%. Because the alloy carbide created in the fire can strengthen it, while the less amount of martensite will decrease the hardness, combining both elements, the hardness changes little after the fire. Mainly due to the decrease of austenite, the tenacity drops a little.The tenacity of the samples are all improved after the isothermal quenching and Q-P and Q-P-T treatment. When it is kept under 300℃ for 5 hours, the tenacity reaches the highest 11.8J, while the hardness drops a lot. After Q-P treatment under 400℃ for 5 minutes, the tenacity can increase 50%, with little drop of hardness, and is suitable to the heat treatment to increase the tenacity of CALDIE steel.