Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 0 1 7 (a) (b) Fig. 1 — Initial microstructure of the steel after continuous cooling through the bainite forming temperature region. Fig. 2 — Details A and B from Fig. 1 showing (a) the fine scale of the bainitic ferrite plates; and (b) relatively coarse retained austenite regions. M achinability is described as the ability of a machining opera- tion to achieve pre-set quality requirements for a given work material, while keeping a pre-defined level of pro- cess efficiency. As such, it can be regard- ed as a system property depending on many variables, including the tool ma- terial, edge geometry, cutting fluid, and characteristics of the work piece. In order to promote formation and breakage of chips, free-machining steels are modified via alloying additions of S, P, Se and sim- ilar elements [1] . Because these additions adversely affect the mechanical proper- ties of the final product, modifications of the microstructure that do not affect its in-service performance are considered themost desirable. The influenece of the microstruc- ture is described with the use of models which correlate the physical properties of the work piece to the machinability index V 60 , as shown in equation 1: (Eq 1) where B = thermal conductivity, L = characteristic length, H B = Brinell hard- ness, A r = the percentage reduction in area obtained during a tensile test. This article describes a procedure for optimizing the annealed microstruc- ture of a high Al KAB steel. The main fo- cus is directed toward characterization of microstructural development during annealing of different phases in a nano- structured bainitic steel using high resolution transmission electron mi- croscopy (HRTEM). EXPERIMENTAL PROCEDURES A steel with nominal composition 0.7C-1.2Si-2.5Mn-2.5Cr-0.6Mo-1.5Al- 0.6(Ni-Cu-W-V), the remainder being iron, was quenched from 940°C in oil and immediately annealed at 680°C, for 24 h. The Brinell hardness (HBW) was measured at 1 h intervals whereby a to- tal of 24 samples were used. The phase fractions were determined using x-ray diffraction on a Bruker D8 Advance. Samples for detailed analysis were taken after 6 h and 24 h as steel reach- es its hardness minimum. For high res- is obtained from an initially bain- itic microstructure [4] . A high defect density is required not only to accel- erate the diffusion controlled growth, but especially due to the fact that the formation of graphite is accompanied by a substantial volume expansion and therefore the graphite particles will fill any available free spaces dur- ing growth [5] . olution transmission electron micros- copy (HRTEM) observations, samples were cut into 3 mm disks of 1 mm thick- ness on a slow speed saw and care- fully ground to a thickness of 100 µm using 1200P SiC paper, followed by ad- ditional thinning using an ion slicer. RESULTS AND DISCUSSION The steel’s initial mi- crostructure has a hard- ness of 58 HRC and is comprised of bainite, mar- tensite, and retained aus- tenite as is seen in Fig. 1. The majority of the ini- tial microstructure is comprised of very fine car- bide-free bainite which is seen in Fig. 2a. Retained austenite is present pre- dominantly in the form of fine blocks as shown in Fig. 2b; its phase fraction was determined by x-ray diffraction (XRD) at roughly 10%. Even though graph- ite is thermodynamically more stable compared to cementite, its obtainment in commonly used steels is very sluggish [2] . It is report- ed that the most suitable temperature for graphite formation is close to 953°K (680°C) [3] , which was there- fore chosen as the anneal- ing temperature. The hard- ness decreased to 290 HBW after 6 h, with no further decrease after holding for 24 h, as seen on the graph in Fig. 3. In addition to tem- perature, the graphitiza- tion kinetics are heavily dependentonthe initialmi- crostructure [4] . When pre- cipitating graphite from pearlitic or martensitic mi- crostructures, the graphite particles are concentrat- ed at the grain boundar- ies and the dispersion is less uniform, whereas the most favorable dispersion