Volume 74, Issue 5

Sintered steel parts are increasingly used for applications subjected to high mechanical loads and therefore have to be heat treated frequently. For manufacturing, the close relationship between composition, esp. the alloy elements used, alloying variant, compactibility and sinterability as well as heat treating response is a characteristic feature. Compared to wrought steels, significantly more parameters have to be considered and defined. On the other hand, the powder metallurgy route offers more freedom regarding material design and property profile. Advanced alloy systems using Cr and/or Mn are demanding on sintering but offer economical and technical advantages, also for heat treatment.

For plastic mold steels, secondary hardening as well as hardenable, corrosion-resistant steels are used. Secondary hardening can be achieved through alloying with Ni, Ti, Al and Cu. The precipitation sequence of combined Cu and intermetallic NiAl particles depends on the precise ratio of the alloying contents. In this study, a dual hardening iron-based alloy with a high ratio of Ni/Cu and Al/Cu and low Carbon content is investigated. These dual hardening steels represent alloys that utilize both carbidic and intermetallic secondary hardening precipitates to achieve secondary hardening. The investigations focus on the initial precipitation of intermetallic particles in order to analyze the sequence of precipitate formation for a specific ratio of alloying elements. DSC measurements were used to identify characteristic precipitation temperatures between 360 °C and 600 °C and the corresponding heat treatment conditions were reproduced using a quenching dilatometer. The initial stages of precipitate formation were examined using high-resolution atom probe tomography. C-enrichments in the form of retained austenite films were evident in the examined material. Heterogeneous nucleation of Cu-NiAl precipitates on these retained austenite films was attributed to exothermal reactions at 460 °C.

Cold massive forming of steel has been used for over 100 years to reduce mechanical finishing for cost reasons. A systematic investigation regarding the austenitic grain coarsening in the following heat treatment steps during the application treatment of e. g. gear steels could not be found, only indications of sporadic qualitative failures. The material 20NiMoCr6-5+Nb for gear shafts was subjected to a simulating treatment for the investigation of cold massive forming. No significant coarse grain growth was observed. Because the degree of cold massive forming at the individual shaft sections was not known, oblique samples were taken from the same bar steel lot, with which degrees of forming could be carried out steplessly from ∊ = 0 to 50 % on a press. After simulation of a high temperature carburization from 990 °C to 1050 °C, no significant coarse grain growth could be observed, only a uniform increase of the grain size index G with increasing temperature at the same holding time. In order to gain further knowledge in the field of modern manufacturing processes on the subject, newly available computer programs should to be used to simulate the production of components from case hardened steels from steel production to the ready to install part and to identify weak points.

In the contact zone of overrolled races high stresses are acting. In order to get a sufficient rolling contact fatigue life, a high hardness in the depth of high material stresses is necessary. One possibility to achieve high hardness in a surface near zone in case of steels is nitriding. By one example of an application to polymer processing and another example of an application to aero engines is demonstrated, how races for overrolling can be realized by nitriding. By these examples the effect of tempering and nitriding parameters on the properties of the nitrided zone as hardness, residual stress and formation of the compound layer is demonstrated. Rig tests show, that overrolling properties of nitrided and case hardened layers can be similar.

Beam welding processes such as electron beam or laser beam welding are used for a range of applications where precise and localised heat input is required. In this contribution, two industrial beam-welding facilities were used to produce joints with a maximum depth of 25 mm. The materials investigated were 42CrMo4 QT-steel and S355 structural steel. The investigations show that both beam welding processes led to comparable results. The butt joints generated were rated by means of macroscopic and microscopic characterization methods. The mechanical properties were investigated by static tensile and Charpy notched bar impact testing. Both beam sources exhibited characteristic features when joining the steels. Due to the smaller beam diameter of the laser, laser welded seams were 40 % to 50 % smaller than their electron beam welded pendants. The reduction of the tensile strength of the initial state was below 4 % for both welding techniques.